What lights or shapes does that vessel show from where you are?

COLREG 1972 Rules 20 to 31 and Annex I as amended to A.1085(28), and the US Inland Rules (33 CFR 83 and 84) · 18 vessels · lights at night from your bearing, shapes by day · Rule 22 minimum ranges by length · rules as of 30 October 2026 · Updated 30 October 2026

Published by ShipCalculators.com. Method checked against COLREG 1972, Rules 20 to 31 and Annex I, 33 CFR Parts 83 and 84 (Inland Rules) and IMO Resolutions A.464(XII) to A.1085(28) on 30 October 2026. How calculators are checked

Answer

A 60 m power-driven vessel making way at night, seen from her starboard beam, shows two white masthead lights, the after one higher, and her green sidelight (COLREG Rules 21 and 23(a)). Under Rule 22(a) the masthead lights carry at least 6 nautical miles and the sidelight 3; from right ahead both sidelights show.

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On this page

Lights and shapes
Calculator
What you see
Cite or share
Drawings
By hand
Step by step
Formula and limits
Arcs of each light
Signals by vessel
US Inland differences
FAQs
Not modeled
Regulatory basis
How to use
Test cases
Change log
Related calculators
Publisher
Sources

What navigation lights and shapes are

Navigation lights and shapes are the signals the International Regulations for Preventing Collisions at Sea, 1972, require a vessel to show so that others can tell what she is and what she is doing: lights from sunset to sunrise and in restricted visibility, shapes by day (Rule 20), black (Annex I, section 6)¹^,⁴. Each light shows only over a set arc of the horizon (Rule 21), so the lights you see depend on your bearing from her bow, and each must carry a minimum range set by her length (Rule 22). The United States applies the same Part C on the high seas and its own Inland Rules, 33 CFR Part 83, inside its waters.²

Find the lights and shapes

Start from an exam situation, or set your own below

1Which rules

2The vessel

Length overall

115012 m20 m50 m100 m

3What she is doing

Arrangement the rule allows

Length of the tow, stern of the tug to the end of the tow

101000200 m

4Where you are

Your bearing from her bow, clockwise

0359112.5°astern247.5°

5Night or day

Under the International Rules (COLREG 1972): a 60 m power-driven vessel, seen from 090°, her starboard beam, at night.

As the rules set out

Power-driven, 60 m, from 090°: 2 white, 1 green

Drag the eye, or swipe the picture, to walk round her.

Her lights by kind

Her lights by kind
Light	Arc	Range	Seen
Masthead	225°	6 nm	2 of 2
Sidelights	112.5°	3 nm	1 of 2
Sternlight	135°	3 nm	0 of 1
Towing light			none
All-round			none

Walking round her, the arcs from above and the ranges

Lights seen: 3 of 5
Bearing: 090°
Masthead light: 6 nm
Sidelights: 3 nm
Sternlight: 3 nm
Towing light: not carried
All-round lights: not carried
Day shapes: 0

Minimum ranges from Rule 22(a), for a vessel 50 m or more; arcs from Rule 21. Positions are schematic.

Seen from her starboard beam (090°) at night, a 60 m power-driven vessel shows two masthead lights (6 nm) and the green sidelight (3 nm).

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What you see and why

Seen from her starboard beam (090°) at night, a 60 m power-driven vessel shows two masthead lights (6 nm) and the green sidelight (3 nm).

She carries five lights under Rule 23(a). The red sidelight and the sternlight are outside their arcs from 090°. For a vessel 50 m or more Rule 22(a) sets the minimum ranges: masthead light 6 miles, sidelights 3, sternlight and all-round lights 3.

Seen from her starboard quarter (112.5°) at night, a 15 m vessel dredging, with an obstruction on one side, making way, shows red, white and red all-round lights in a vertical line (2 nm); a masthead light (3 nm); the green sidelight (2 nm); two all-round red lights on the obstruction side (2 nm); and two all-round green lights on the passing side (2 nm).

She carries eleven lights under Rule 27(d) of the Inland Rules. The red sidelight and the sternlight are outside their arcs from 112.5°. For a vessel 12 m to under 20 m Rule 22(b) sets the minimum ranges: masthead light 3 miles, sidelights 2, sternlight and all-round lights 2.

Seen from her starboard quarter (112.5°) at night, a 15 m vessel restricted in her ability to maneuver, making way, shows red, white and red all-round lights in a vertical line (2 nm); a masthead light (3 nm); and the green sidelight (2 nm).

She carries seven lights under Rule 27(b) of the Inland Rules. The red sidelight and the sternlight are outside their arcs from 112.5°. For a vessel 12 m to under 20 m Rule 22(b) sets the minimum ranges: masthead light 3 miles, sidelights 2, sternlight and all-round lights 2.

Seen from her starboard quarter (112.5°) at night, a 120 m power-driven vessel towing astern shows three masthead lights in a vertical line (6 nm), the after masthead light (6 nm) and the green sidelight (3 nm).

She carries eight lights under Rule 24(a) of the Inland Rules. The red sidelight, the sternlight and the yellow towing light are outside their arcs from 112.5°. For a vessel 50 m or more Rule 22(a) sets the minimum ranges: masthead light 6 miles, sidelights 3, sternlight and all-round lights 3.

Cite or share this result

ShipCalculators.com, “COLREG Navigation Lights and Shapes Calculator.” Method: the lights and shapes of COLREG Rules 23 to 30 (and 33 CFR 83 for the Inland Rules), each light shown when your bearing is inside its Rule 21 arc, its minimum range from Rule 22. Inputs: Under the International Rules (COLREG 1972): a 60 m power-driven vessel, seen from 090°, her starboard beam, at night. Result: Seen at night: 2 white, 1 green, from 090°, her starboard beam. https://shipcalculators.com/calculators/colregs-lights-and-shapes

ShipCalculators.com, “COLREG Navigation Lights and Shapes Calculator.” Method: the lights and shapes of COLREG Rules 23 to 30 (and 33 CFR 83 for the Inland Rules), each light shown when your bearing is inside its Rule 21 arc, its minimum range from Rule 22. Inputs: Under the US Inland Rules, Western Rivers: an 11.5 m fishing vessel trawling, underway, stopped, after masthead light shown (optional under 50 m), seen from 112.5°, her starboard quarter, at night. Result: Seen at night: 2 white, 1 green, from 112.5°, her starboard quarter. https://shipcalculators.com/calculators/colregs-lights-and-shapes?rs=inland&vt=trawl&L=11.5&st=stop&b=112.5&al=two&wa=wr

ShipCalculators.com, “COLREG Navigation Lights and Shapes Calculator.” Method: the lights and shapes of COLREG Rules 23 to 30 (and 33 CFR 83 for the Inland Rules), each light shown when your bearing is inside its Rule 21 arc, its minimum range from Rule 22. Inputs: Under the US Inland Rules, Western Rivers: an 11.5 m vessel dredging, with an obstruction on one side, underway, stopped, seen from 112.5°, her starboard quarter, by day. Result: By day: Ball, diamond, ball + 4 more, from 112.5°, her starboard quarter. https://shipcalculators.com/calculators/colregs-lights-and-shapes?rs=inland&vt=dredge&L=11.5&st=stop&b=112.5&dn=day&sd=stbd&wa=wr

ShipCalculators.com, “COLREG Navigation Lights and Shapes Calculator.” Method: the lights and shapes of COLREG Rules 23 to 30 (and 33 CFR 83 for the Inland Rules), each light shown when your bearing is inside its Rule 21 arc, its minimum range from Rule 22. Inputs: Under the US Inland Rules, Western Rivers: an 11.5 m fishing vessel not trawling, underway, stopped, gear out over 150 m, seen from 112.5°, her starboard quarter, by day. Result: By day: Two cones + 1 more, from 112.5°, her starboard quarter. https://shipcalculators.com/calculators/colregs-lights-and-shapes?rs=inland&vt=fish&L=11.5&st=stop&b=112.5&dn=day&al=gear&sd=stbd&wa=wr

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The vessel drawn from every side

Walking round her, 360°

Walking round her: 090°, stbd beam. Tap the pictures to pause.

Each light's arc, from above

Minimum ranges

Seen from above, bow up: forward masthead light 225°, after masthead light, higher 225°, green sidelight, starboard 112.5°, red sidelight, port 112.5°, sternlight 135°. You are at 090°, her starboard beam.

Seen from above, bow up: all-round red, top 360°, all-round white, middle 360°, all-round red, bottom 360°, forward masthead light 225°, after masthead light, higher 225°, green sidelight, starboard 112.5°, red sidelight, port 112.5°, sternlight 135°, all-round red, obstruction side, upper 360°, all-round red, obstruction side, lower 360°, all-round green, passing side, upper 360°, all-round green, passing side, lower 360°. You are at 112.5°, her starboard quarter.

The vessel is drawn as her kind, not to scale, and her lights stand where Annex I puts them in height order, not at its distances. An asterisk marks a light or shape the rule allows but does not require. Point at a light, or tap it, to read its rule.

Reading a vessel's lights by hand

Start from the rule for what she is doing: Rule 23 for a power-driven vessel, 24 towing, 25 sailing, 26 fishing, 27 not under command or restricted, 28 constrained by her draft, 29 pilot, 30 at anchor or aground. List the lights it gives her. Then take your bearing from her bow, clockwise. The masthead light shows from 247.5° through right ahead to 112.5°, the green sidelight from 0° to 112.5°, the red from 247.5° to 360°, the sternlight and any towing light from 112.5° to 247.5°, and all-round lights from everywhere. Keep the lights whose arc holds your bearing. Their ranges come from Rule 22 by her length: 6 miles for a masthead light at 50 m or more, 5 from 20 m, 3 from 12 m, 2 below; sidelights 3 miles at 50 m or more, 2 from 12 m, 1 below; the sternlight and all-round lights 3 miles at 50 m or more, 2 below.

Your case step by step

Length 60 m: Rule 22(a), a vessel 50 m or more. Masthead 6 nm, sidelight 3 nm, sternlight 3 nm, towing and all-round 3 nm.
Lights carried, Rule 23(a): two masthead lights, the green sidelight, the red sidelight and the sternlight.
Your bearing 090° from her bow: her starboard beam. Inside the masthead light's 225° (247.5° through 0° to 112.5°); inside the green sidelight's 0° to 112.5°. All-round lights show from every bearing.
Seen: two masthead lights and the green sidelight, which reads 2 white, 1 green. By day: no shape.

Length 120 m: Rule 22(a), a vessel 50 m or more. Masthead 6 nm, sidelight 3 nm, sternlight 3 nm, towing and all-round 3 nm.
Lights carried, Rule 27(d): red, white and red all-round lights in a vertical line; two masthead lights; the green sidelight; the red sidelight; the sternlight; two all-round red lights on the obstruction side; and two all-round green lights on the passing side.
Your bearing 112.5° from her bow: her starboard quarter. Inside the masthead light's 225° (247.5° through 0° to 112.5°); inside the green sidelight's 0° to 112.5°. All-round lights show from every bearing.
Seen: red, white and red all-round lights in a vertical line; two masthead lights; the green sidelight; two all-round red lights on the obstruction side; and two all-round green lights on the passing side, which reads 3 white, 4 red, 3 green. By day: ball, diamond, ball + 4 more.

Length 120 m: Rule 22(a), a vessel 50 m or more. Masthead 6 nm, sidelight 3 nm, sternlight 3 nm, towing and all-round 3 nm.
Lights carried, Rule 28: two masthead lights, the green sidelight, the red sidelight, the sternlight and three all-round red lights in a vertical line; all-round red, top; all-round red, middle; and all-round red, bottom optional.
Your bearing 112.5° from her bow: her starboard quarter. Inside the masthead light's 225° (247.5° through 0° to 112.5°); inside the green sidelight's 0° to 112.5°. All-round lights show from every bearing.
Seen: two masthead lights, the green sidelight and three all-round red lights in a vertical line, which reads 2 white, 3 red, 1 green. By day: cylinder.

Length 15 m: Rule 22(b), a vessel 12 m to under 20 m. Masthead 3 nm, sidelight 2 nm, sternlight 2 nm, towing and all-round 2 nm.
Lights carried, Rule 26(b): all-round green over white lights, a masthead light, the green sidelight, the red sidelight and the sternlight; masthead light, abaft and higher than the green optional.
Your bearing 000° from her bow: right ahead. Inside the masthead light's 225° (247.5° through 0° to 112.5°); both sidelights show right ahead. All-round lights show from every bearing.
Seen: all-round green over white lights, a masthead light, the green sidelight and the red sidelight, which reads 2 white, 1 red, 2 green. By day: two cones.

Formula, assumptions, and limits

What a vessel shows depends on three things the reader sets: the rule that applies to her (Rules 23 to 31), her length overall, and the bearing she is seen from. The rule decides which lights she carries, Rule 21 decides from which bearings each one can be seen, and Rule 22 decides how far off each one must carry.

Formula

A light is seen when your bearing from her bow, $\theta$ , measured clockwise, falls inside its arc:

\begin{aligned} \text{masthead: } & \theta \ge 247.5^\circ \\ & \text{or } \theta \le 112.5^\circ \\ \text{green: } & \theta \le 112.5^\circ \\ \text{red: } & \theta \ge 247.5^\circ \\ \text{stern: } & 112.5^\circ < \theta \\ & \theta < 247.5^\circ \end{aligned}

The yellow towing light has the sternlight’s arc.

The minimum range of the masthead light, in nautical miles, by length overall $L$ in meters (Rule 22):

R_{\text{mh}} = \begin{cases} 6 & L \ge 50 \\ 5 & 20 \le L < 50 \\ 3 & 12 \le L < 20 \\ 2 & L < 12 \end{cases}

Annex I, section 8, turns a range into the luminous intensity a lantern needs:

I = 3.43 \times 10^{6} \, T \, D^{2} \, K^{-D}

$\theta$ : the observer’s bearing from the vessel’s bow, clockwise, 0° to 360°.
$L$ : length overall, the definition in Rule 3(j).
$R_{\text{mh}}$ : the masthead light’s minimum range. The sidelights carry 3, 2, 2 and 1 miles in the same four bands; the sternlight, towing light and all-round lights 3 miles at 50 m or more and 2 miles below.
$I$ : luminous intensity in candelas; $T = 2 \times 10^{-7}$ lux; $D$ the range in nautical miles; $K = 0.8$ , the atmospheric transmissivity Annex I sets for prescribed lights.

Derivation

Rule 21(a) defines the masthead light as a white light “showing an unbroken light over an arc of the horizon of 225° and so fixed as to show the light from right ahead to 22.5° abaft the beam on either side of the vessel.” Right ahead is 0°, the starboard beam 90°, so 22.5° abaft it is 112.5°; on the port side the arc ends at 247.5°. Each sidelight covers 112.5° on its own side, and the sternlight the 135° between them, “67.5° from right aft on each side.” The arcs of the masthead light and the sternlight together close the horizon, so a power-driven vessel shows her masthead lights or her sternlight from every bearing, never both.

Assumptions

The vessel carries what her rule requires, and the optional lights only where the reader picks the arrangement that shows them; they are marked with an asterisk.
At exactly 112.5° and 247.5° the calculator shows the light on the bow side and not the sternlight. Annex I, section 9, lets the intensity of a real lantern fall off over up to 5° beyond its arc, so near those bearings both may show.
Lights are drawn on a schematic ship. Their heights keep the order Annex I sets (the after masthead light higher than the forward one, the sidelights below the masthead light, the after anchor light lower than the forward one), not its distances.
Length overall is the length the rules use for every band.

Worked example

A 60 m power-driven vessel making way at night carries a forward masthead light, an after masthead light higher than it, sidelights and a sternlight, Rule 23(a). Seen from her starboard beam, 90°, the bearing is inside the masthead lights’ arc (up to 112.5°) and the green sidelight’s (0° to 112.5°), outside the red sidelight’s and the sternlight’s. You see two white lights, the after one higher, and a green light below them. At 50 m or more Rule 22(a) puts the masthead lights at 6 miles and the sidelight at 3. From right ahead you would see both masthead lights in line and both sidelights; from right astern, only the sternlight. Annex I, section 8, gives the intensity those ranges need: 94 candelas for 6 miles and 12 for 3.

Edge cases and limits

Tows over 200 m. A towing vessel shows two masthead lights in a vertical line instead of her forward masthead light, three when the tow measured from her stern to the after end of the tow exceeds 200 m, and a yellow towing light above her sternlight (Rule 24(a)). The calculator draws the vertical lights forward and the after masthead light from 50 m.
Small craft. Under 12 m a power-driven vessel may show an all-round white light and sidelights; under 7 m and at no more than 7 knots, an all-round white light alone (International Rules only). Under 12 m a vessel need not show the signals of Rule 27, except a vessel engaged in diving operations.
Not modeled. Air-cushion vessels, WIG craft and seaplanes, the fishing signals of Annex II, inconspicuous partly submerged objects under tow, dredge pipelines, and the Inland law enforcement and public safety lights (33 CFR 88.05 and 88.07).

Regulatory basis

The International Regulations for Preventing Collisions at Sea, 1972, Part C, Rules 20 to 31, and Annex I, as amended by IMO Resolutions A.464(XII), A.626(15), A.736(18), A.910(22) and A.1085(28), the last in force from 1 January 2016. The text is quoted from the US Coast Guard Navigation Rules Handbook, corrected 8 August 2024. The US Inland Rules are 33 CFR Part 83 and their Annex I is Part 84, as in force on 1 October 2026.

Common errors

These are contrasts drawn from the rule text, each a pair of signals that differ in one light or one shape.

Pilot vessel as red over white. A pilot vessel on duty shows white over red (Rule 29); red over white is a fishing vessel not trawling (Rule 26(c)).
Aground as three red. A vessel aground shows two all-round red lights where they can best be seen, with her anchor lights (Rule 30(d)); three red in a vertical line is a vessel constrained by her draft (Rule 28).
Masthead lights on a vessel not under command. Rule 27(a) gives her two red lights and, when making way, sidelights and a sternlight, and no masthead light.
A diamond on a short tow. The diamond is shown only when the tow exceeds 200 m.
Anchor lights on a fishing vessel at anchor. Rule 26(a) says she shows only the lights and shapes of Rule 26, at anchor or not.

Arcs of each light

Light	Color	Arc	From your bearing
Masthead light	White	225°	247.5° through 0° to 112.5°
Starboard sidelight	Green	112.5°	0° to 112.5°
Port sidelight	Red	112.5°	247.5° to 360°
Sternlight	White	135°	112.5° to 247.5°
Towing light	Yellow	135°	112.5° to 247.5°
All-round light	Any	360°	every bearing
Special flashing light, Inland	Yellow	180° to 225°	ahead to 22.5° abaft the beam

Lights and shapes by vessel

Read across from the lights you see to the vessel showing them; a calculator mode that starts from the lights seen is planned.

Vessel	Lights at night	Shape by day	Rule
Power-driven, underway	Masthead (two at 50 m or more), sidelights, sternlight	none	23(a)
Towing astern	Two masthead in a vertical line (three over 200 m), sidelights, sternlight, yellow towing light above it	diamond over 200 m	24(a)
Sailing	Sidelights, sternlight; tricolor under 20 m	none (cone, apex down, when also under power)	25
Trawling	Green over white all-round; masthead abaft at 50 m or more	two cones, apexes together	26(b)
Fishing, not trawling	Red over white all-round	two cones, apexes together	26(c)
Not under command	Two red all-round	two balls	27(a)
Restricted in ability to maneuver	Red, white, red all-round	ball, diamond, ball	27(b)
Constrained by her draft	Three red all-round (optional)	cylinder (optional)	28, International only
Pilot vessel on duty	White over red all-round	none prescribed	29
At anchor	All-round white forward, a lower one aft	one ball	30(a)
Aground	Anchor lights and two red all-round	three balls	30(d)

Making way through the water adds sidelights and a sternlight to the fishing and not under command signals, and masthead lights, sidelights and a sternlight to the restricted signals (Rule 27(b)(iii)); a pilot vessel shows sidelights and a sternlight whenever she is underway.

Where the US Inland Rules differ

The Inland Rules follow the International Rules light for light, with these exceptions a watchkeeper can see from another vessel:

No constrained-by-draft signal. 33 CFR 83.28 is reserved.
Pushing ahead or towing alongside. Two yellow towing lights in a vertical line replace the sternlight (83.24(c)), and the tow pushed ahead carries a special flashing yellow light at 50 to 70 flashes a minute, visible 2 miles. On the Western Rivers above the Huey P. Long Bridge the pushing vessel shows no masthead lights at all, only sidelights and the two towing lights (83.24(j)).
Great Lakes. A power-driven vessel may carry an all-round white light in place of her second masthead light and sternlight, at the masthead light’s range (83.23(e)).
Small craft and anchorages. The 7 m, 7 knot option does not exist, and a vessel under 20 m in a special anchorage the Coast Guard designates need not show anchor lights (83.30(g)).
Annex I spacing. Masthead lights may be closer together: 2 m apart vertically and a quarter of her length horizontally (Part 84).

Frequently Asked Questions (FAQs)

What lights does a power-driven vessel show at night?

A masthead light forward, a second masthead light abaft and higher than the first, sidelights and a sternlight, under COLREG Rule 23(a). A vessel under 50 m need not show the second masthead light but may. Under 12 m she may show an all-round white light and sidelights instead.

What are the arcs of the masthead light, sidelights and sternlight?

Rule 21 sets them. The masthead light shows over 225°, from right ahead to 22.5° abaft the beam on either side. Each sidelight shows over 112.5° from right ahead to 22.5° abaft the beam on its own side, green to starboard and red to port. The sternlight shows over 135°, 67.5° either side of right astern.

How far must navigation lights be visible?

Rule 22 sets minimum ranges by length. At 50 m or more: masthead light 6 miles, sidelights, sternlight, towing light and all-round lights 3 miles. From 12 m to under 50 m: masthead 5 miles (3 under 20 m), the others 2. Under 12 m: masthead 2 miles, sidelights 1, the others 2.

What do red over white and white over red lights mean?

Red over white, two all-round lights in a vertical line, is a vessel engaged in fishing other than trawling (Rule 26(c)); by day she shows two cones with their apexes together. White over red is a pilot vessel on pilotage duty (Rule 29). Green over white is a vessel trawling (Rule 26(b)).

What lights does a sailing vessel show?

Sidelights and a sternlight, under Rule 25(a). Under 20 m she may combine them in one tricolor lantern at or near the masthead, and any sailing vessel may add all-round red over green at the masthead, but never with the tricolor. When she is also under power she is a power-driven vessel, and by day shows a cone, apex downward (optional under 12 m on US Inland waters).

What do two red lights in a vertical line mean?

A vessel not under command, Rule 27(a), and by day two balls. She adds sidelights and a sternlight when making way through the water, but never masthead lights. Two all-round red lights in a vertical line shown with her anchor lights mean she is aground, Rule 30(d), and by day she shows three balls.

When does a towing vessel show a diamond shape?

When the tow is longer than 200 m, measured from the stern of the towing vessel to the after end of the tow. Rule 24(a) then requires three masthead lights in a vertical line instead of two, and a diamond shape on the towing vessel and on the tow by day.

Does constrained by draft apply under the US Inland Rules?

No. The International Rules let a vessel constrained by her draft show three all-round red lights in a vertical line or a cylinder (Rule 28), but the Inland Rules have no such category: 33 CFR 83.28 is reserved. Under the Inland Rules she shows the lights of a power-driven vessel.

What lights does a vessel at anchor show?

An all-round white light in the fore part and a second, lower all-round white light at or near the stern, and one ball by day (Rule 30(a)). Under 50 m she may show a single all-round white light where it can best be seen. At 100 m or more she must also light her decks.

What the calculator does not show

Air-cushion vessels, WIG craft and seaplanes (Rules 23(b), 23(c) and 31), the additional fishing signals of Annex II, inconspicuous partly submerged objects under tow (Rule 24(g)), and dredge pipelines (33 CFR 83.27(d)(iv)).
The Inland law enforcement blue light and the public safety red and yellow lights (33 CFR 88.05 and 88.07).¹¹
Sound signals in restricted visibility (Rule 35), distress signals (Annex IV) and the lights a vessel shows when she should not.
Exact positions: lights are drawn on a schematic ship in the height order Annex I sets, not at its distances.³
Identifying a vessel from lights already seen: a second phase of this calculator.

Regulatory basis

The lights and shapes are those of COLREG 1972 Part C and Annex I as amended by IMO Resolutions A.464(XII), A.626(15), A.736(18), A.910(22) and A.1085(28),⁴^,⁵^,⁶^,⁷^,⁸^,⁹ quoted from the US Coast Guard Navigation Rules Handbook, corrected 8 August 2024.¹ The US Inland Rules are 33 CFR Parts 83 and 84 as in force on 1 October 2026,²^,³ with the Western Rivers and the other waters named in Part 89.¹²

Arcs: Masthead 225°, sidelights 112.5° each, sternlight and towing light 135°, all-round 360°, Rule 21¹
Ranges: Masthead 6, 5, 3 or 2 miles by length band; sidelights 3, 2 or 1; the others 3 or 2, Rule 22¹
Intensity: I = 3.43 × 10^6 × T × D² × K^-D, T = 2 × 10^-7 lux, K = 0.8, Annex I section 8¹^,³
Last amendment: A.1085(28), in force 1 January 2016; no lights change since A.910(22), 2003⁸^,⁹
Towing over 200 m: Three masthead lights in a vertical line and a diamond, Rule 24(a)¹
Constrained by draft: Three red all-round or a cylinder, International only; 33 CFR 83.28 reserved¹^,²
Inland pushing ahead: Two towing lights for the sternlight; a special flashing yellow light on the tow; Western Rivers, no masthead lights, 33 CFR 83.24²

How to use this calculator

Pick the International Rules or the US Inland Rules; for Inland, pick the Great Lakes, the Western Rivers or other Inland waters where the rules differ.
Pick the vessel: power-driven, towing or pushing, sailing, fishing, not under command, restricted in her ability to maneuver, constrained by her draft, a pilot vessel, at anchor or aground, among 18.
Set her length overall, which decides the Rule 22 ranges and some lights she may leave out, and what she is doing: making way, stopped or at anchor, with the tow length for a tow.
Set your bearing from her bow, clockwise: 0° right ahead, 90° on her starboard beam, 180° right astern.
Choose night or day, then read the lights seen with their ranges, the shapes by day, the lights hidden from your bearing and the rules that set them.

Test cases this calculator passes

Every build of this site runs these cases through the calculator above and stops the release if any figure moves outside its tolerance. Load a case to put its inputs into the calculator.

Inputs	Expected result	Traced to	Load
60 m power-driven vessel making way, seen from 090° (starboard beam), at night, International Rules	Lights seen: 3 of 5Masthead light: 6 nmSidelights: 3 nmSternlight: 3 nm	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 23(a): two masthead lights (L ≥ 50 m), sidelights, sternlight; Rule 21: at 090° the masthead (to 112.5°) and green (0° to 112.5°) arcs hold, red and stern do not; Rule 22(a): masthead 6 nm, sidelight 3 nm, sternlight 3 nm.	Load inputs
Same vessel from right ahead, 000°	Lights seen: 4 of 5	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 21(a), (b): both masthead lights and both sidelights show right ahead; the sternlight (112.5° to 247.5°) does not.	Load inputs
Same vessel from right astern, 180°	Lights seen: 1 of 5	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 21(c): only the sternlight, 67.5° either side of right aft.	Load inputs
Same vessel at exactly 112.5°, 22.5° abaft the starboard beam	Lights seen: 3 of 5	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 21(a), (b) arcs end at 22.5° abaft the beam; this page shows the bow-side lights at the boundary (Annex I section 9 allows up to 5° of overlap).	Load inputs
Power-driven vessel of exactly 50 m	Masthead light: 6 nmSidelights: 3 nm	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 22(a), vessels of 50 m or more: masthead 6 miles, sidelight 3 miles.	Load inputs

The other 21 test cases

Inputs	Expected result	Traced to	Load
Power-driven vessel of 40 m	Masthead light: 5 nmSidelights: 2 nmSternlight: 2 nmLights seen: 2 of 4	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 22(b), 12 m to under 50 m: masthead 5 miles, sidelight 2, sternlight 2; Rule 23(a)(ii): no second masthead light required under 50 m.	Load inputs
Power-driven vessel of exactly 20 m	Masthead light: 5 nm	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 22(b): the 3-mile masthead range applies only where the length is less than 20 m, so 5 miles at 20 m.	Load inputs
Power-driven vessel of exactly 12 m	Masthead light: 3 nmSidelights: 2 nm	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 22(b), 12 m or more and less than 20 m: masthead 3 miles, sidelight 2 miles.	Load inputs
Power-driven vessel of 11.9 m	Masthead light: 2 nmSidelights: 1 nmSternlight: 2 nm	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 22(c), under 12 m: masthead 2 miles, sidelight 1 mile, sternlight 2 miles.	Load inputs
Tug of 30 m towing astern, tow 250 m, seen from right astern	Lights seen: 2 of 7Towing light: 2 nmDay shapes: 1	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 24(a): three masthead lights (tow over 200 m), sidelights, sternlight, yellow towing light above it, diamond by day; from astern only the sternlight and the towing light; Rule 22(b): towing light 2 miles.	Load inputs
300 m vessel constrained by her draft, right ahead, International Rules	Lights seen: 7 of 8All-round lights: 3 nmDay shapes: 1	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 28: three all-round red lights in a vertical line or a cylinder, in addition to Rule 23 lights; Rule 22(a): all-round 3 miles.	Load inputs
Same vessel under the US Inland Rules	Lights seen: 4 of 5All-round lights: not carriedDay shapes: 0	33 CFR 83.28 [Reserved]: no constrained-by-draft signal under the Inland Rules; power-driven lights only.	Load inputs
120 m vessel not under command, making way, seen from 045°	Lights seen: 3 of 5Masthead light: not carriedDay shapes: 2	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 27(a): two all-round red lights, sidelights and sternlight when making way, no masthead light; two balls by day.	Load inputs
80 m vessel aground, seen from 270°	Lights seen: 4 of 4All-round lights: 3 nmDay shapes: 3	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 30(d): anchor lights (Rule 30(a)) and two all-round red lights; three balls by day; Rule 22(a): all-round 3 miles.	Load inputs
20 m pilot vessel on duty at anchor, seen from 090°	Lights seen: 4 of 4Sidelights: not carriedDay shapes: 1	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 29(a): white over red at the masthead and, at anchor, the anchor lights of Rule 30, no sidelights; one ball by day.	Load inputs
9 m sailing vessel with a tricolor, seen from 315°	Lights seen: 1 of 3Sidelights: 1 nmSternlight: 2 nm	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 25(b): under 20 m sidelights and sternlight may be combined at the masthead; Rule 22(c): sidelight 1 mile, sternlight 2 miles; from 315° only the red sector.	Load inputs
40 m vessel pushing ahead on the Western Rivers, right ahead, US Inland Rules	Lights seen: 2 of 4Masthead light: not carriedTowing light: 2 nm	33 CFR 83.24(j): on the Western Rivers a vessel pushing ahead shows sidelights and two towing lights in a vertical line, no masthead lights; 83.22(b): towing light 2 miles.	Load inputs
40 m trawler at anchor, seen from 090°	Lights seen: 2 of 2Sidelights: not carriedDay shapes: 1	USCG Navigation Rules Handbook (corrected 8 Aug 2024): Rule 26(a): a vessel engaged in fishing, underway or at anchor, shows only the lights and shapes of Rule 26: green over white, no sidelights unless making way.	Load inputs
Great Lakes light chosen on other Inland waters, 60 m power-driven vessel from right astern	Lights seen: 1 of 5	33 CFR 83.23(e): the all-round white light in place of the after masthead light and sternlight is for a vessel on the Great Lakes only; elsewhere she shows the standard lights, so only the sternlight is seen from astern.	Load inputs
60 m power-driven vessel on the Great Lakes with the all-round white light, from her starboard beam	Lights seen: 3 of 4All-round lights: 6 nm	33 CFR 83.23(e): on the Great Lakes an all-round white light may replace the second masthead light and the sternlight; from the beam she shows the forward masthead light, the green sidelight and the all-round white.	Load inputs
10 m vessel not under command, making way, from her starboard beam	Lights seen: 3 of 5	Rule 27(a) and (g): two all-round red, sidelights and sternlight, five lights; from the beam the two reds and the green show. Under 12 m she need not show them, so the result carries the note.	Load inputs
60 m vessel pushing ahead on other Inland waters, from right astern	Lights seen: 2 of 7Towing light: 3 nm	33 CFR 83.24(c): two masthead lights forward, an after masthead light at 50 m or more, sidelights and two yellow towing lights in place of the sternlight; from astern only the two towing lights show.	Load inputs
80 m vessel restricted in her ability to maneuver, at anchor, by day	Day shapes: 4	Rule 27(b)(ii) and (iv): ball, diamond, ball, and the anchor ball.	Load inputs
60 m power-driven vessel seen from 247.5°, the port side's arc limit	Lights seen: 3 of 5	Rule 21: at exactly 22.5° abaft the beam the masthead lights and the red sidelight show, the sternlight does not (the bow-side convention stated in the note).	Load inputs
Length left blank	Shows “Enter the length overall in meters, from 1 to 500.”	Refused: the length overall decides the Rule 22 band.	Load inputs
Bearing left blank	Shows “Enter your bearing from her bow, clockwise, from 0 to 360 degrees.”	Refused: the bearing decides which lights are inside their Rule 21 arcs.	Load inputs

Change log

Each change to the method, a coefficient or a test case, with the date it reached this page.

30 October 2026 First published, replacing the vessel light identification and light visibility range calculators: 18 vessels under the International Rules and the US Inland Rules, each light drawn from the reader’s bearing with its Rule 21 arc and Rule 22 range, and the day shapes.

Calculators used with this one

The sea distance calculator gives the passage in nautical miles, the unit every light range here is stated in.

The rate of turn calculator works out the turn a vessel makes once her lights tell you to keep clear.

The ship ETA calculator gives the time of arrival for the same passage.

Who publishes this calculator and how it is checked

Publisher: ShipCalculators.com, an independent site of maritime calculators.
How it is checked: Each figure is traced to the instrument that sets it, and the test cases on this page run against the calculator code on every build; a build that moves one of them does not deploy. Last method check: 30 October 2026.
What it is not: Not a lights plan or a survey: a ship's lights are fitted to the satisfaction of the authority of her flag State (Annex I, section 14), and the Rules themselves are the authority on what a vessel must show.

What navigation lights and shapes are

Find the lights and shapes

What you see and why

Cite or share this result

The vessel drawn from every side

Walking round her, 360°

Each light's arc, from above

Minimum ranges

Reading a vessel's lights by hand

Your case step by step

Formula, assumptions, and limits

Formula

Derivation

Assumptions

Worked example

Edge cases and limits

Regulatory basis

Common errors

Arcs of each light

Lights and shapes by vessel

Where the US Inland Rules differ

Frequently Asked Questions (FAQs)

What lights does a power-driven vessel show at night?

What are the arcs of the masthead light, sidelights and sternlight?

How far must navigation lights be visible?

What do red over white and white over red lights mean?

What lights does a sailing vessel show?

What do two red lights in a vertical line mean?

When does a towing vessel show a diamond shape?

Does constrained by draft apply under the US Inland Rules?

What lights does a vessel at anchor show?

What the calculator does not show

Regulatory basis

How to use this calculator

Test cases this calculator passes

Change log

Calculators used with this one

Who publishes this calculator and how it is checked

Regulation and sources