How far is it between two ports by sea?

Formula, assumptions, and limits

The sea distance is the length of the shortest path between two ports along a network of shipping lanes, plus the leg from each port to the network. Every segment is a geodesic on the WGS 84 ellipsoid, measured in nautical miles of exactly 1,852 m, the value in Table 8 of the BIPM’s SI Brochure, 9th edition. Time at sea is that distance divided by the speed in knots.

Formula

• $D$: the sea distance, nautical miles.
• $d_A$, $d_B$: the leg from the departure and the arrival port to the lane network, nautical miles; a straight geodesic where it stays at sea, and a path around the land where it does not.
• $s_i$: the geodesic length of lane segment $i$, from network point $P_i$ to $P_{i+1}$, by Vincenty’s inverse method on WGS 84.
• $D_{ECA}$: the part of $D$ inside the emission control areas in force on the sailing date, nautical miles; $E$ is the set of route pieces inside one, and $s_j$ the length of piece $j$.
• $V$: the speed over the ground for the passage, knots.
• $t$: hours at sea; $T_0$ the departure in UTC; $h$ any hours the reader adds for canal transits and waiting.

Derivation

A knot is one nautical mile an hour, so hours at sea are miles over knots. The route comes from Dijkstra’s shortest-path search over the Eurostat SeaRoute maritime network (MARNET) at its 5 km resolution: 42,322 points and 78,683 lane segments, built by Eurostat from the Oak Ridge National Laboratory global shipping lane network and extended with lanes around the European coasts from AIS tracks. Segments through the Suez, Panama, Kiel and Corinth canals, the Straits of Gibraltar, Dover, Malacca, Bab-el-Mandeb, Bering and Magellan, and the Northwest and Northeast passages carry a passage tag, and the search leaves out a tagged segment when the reader closes that canal. The Northwest and Northeast passages are closed by default.

Port positions are those of the National Geospatial-Intelligence Agency’s World Port Index (Pub. 150): the 3,630 ports of its 2019 database, with each position replaced by the current index’s where NGA has revised it, and 10 ports the index has added since. NGA Pub. 151, Distances Between Ports, adds 105 ports more than 10 nautical miles from any index port, and Wikidata adds Duqm and Khalifa Port, which neither NGA source lists. A port joins the network at its nearest lane point when the straight leg stays at sea. Where it crosses land, the leg follows a path around it on a 0.01° grid drawn from the Natural Earth coastline.

The emission control area split tests each lane segment against the area boundaries, cut where a segment crosses one, and counts only areas whose 0.10% sulphur limit applies on the sailing date.

Assumptions

• The ship follows the lane network, which is a model of where ships sail, not a passage plan.
• One speed for the whole passage, over the ground.
• Canal transits count at their lane length; time in the canal and at anchor waiting for a convoy is the reader’s own figure.
• Each port is a single point, the World Port Index position, not a berth.

Worked example

Rotterdam to Singapore, sailing 2 March 2026 at 08:00 UTC at 14 knots, with every canal open. The route runs down the English Channel, through the Strait of Gibraltar, the Mediterranean, the Suez Canal, Bab-el-Mandeb and the Strait of Malacca.

1. Network length 8,409.30 nautical miles; Rotterdam lies 0.47 nautical miles from the network and Singapore 1.27, so $D$ = 8,411.04 nautical miles.
2. $t$ = 8,411.04 ÷ 14 = 600.79 hours, 25 days 47 minutes.
3. $ETA$ = 2 March 08:00 UTC + 600 hours 47 minutes = 27 March 2026 at 08:47 UTC.
4. Inside emission control areas in force on 2 March 2026: 420.95 nautical miles in the North Sea area, from Rotterdam to 5° W in the English Channel, and 1,941.37 in the Mediterranean, from the line Cape Trafalgar to Cape Spartel to the Suez Canal’s northern entrance. $D_{ECA}$ = 2,362.32 nautical miles, 168.7 hours at 14 knots; the other 6,048.72 are outside.

The Suez Canal Authority gives this pair as 8,288 nautical miles, so the calculator reads 1.5% long, inside the band measured on the 20 ocean routes. With the Suez Canal closed the route goes around the Cape of Good Hope: 11,909.59 nautical miles, 850.7 hours at 14 knots, 10.4 days more.

Edge cases and limits

• Short coastal legs. On 12 pairs under 300 nautical miles the median difference from NGA and NOAA was 15.8%; Charleston to Savannah came out 50% long, with the network’s lane well off the coast and Savannah reached up its river. The result box flags any route under 300 nautical miles.
• Ports far from the lanes. The distance from each port to the network shows under the result. Past 25 nautical miles the result is flagged; 1,088 of the 3,493 ports are that far, and for them the leg is the least certain part of the distance.
• Great Lakes and inland ports. 229 ports, 175 of them on the Great Lakes, have no sea path to the network on the Natural Earth coastline and are left out. NGA Pub. 151 also leaves the Great Lakes to the US Coast Pilot.
• Draft, beam and air draft. The network does not know a ship’s size. A ship too large for the Panama Canal’s locks, or too deep for the Suez Canal, still gets the canal route unless the canal is switched off.

Regulatory basis

No convention prescribes how a sea distance is measured. The nautical mile is the 1,852 m of the International Hydrographic Conference of 1929, listed by the BIPM. The emission control areas are those of MARPOL Annex VI Regulation 14 and its Appendix VII, with the 0.10% sulphur limit of Regulation 14.4 and the 12-month exemption of Regulation 14.7 for a newly designated area, both in the text adopted by resolution MEPC.328(76).

Common errors

• Quoting a network distance as a charter-party distance. Charter parties name their own table, and this calculator is not that table.
• Adding the canal length a second time. The lane through the Suez Canal is already part of the route; 193.30 km is the Suez Canal Authority’s figure for the whole canal, 104.4 nautical miles.
• Dividing by speed through the water on a passage with a known current. Hours at sea follow the speed over the ground.
• Counting the Mediterranean emission control area before 1 May 2025, or the Canadian Arctic and Norwegian Sea before 1 March 2027.

How accurate a network distance is

A network distance is close on ocean passages and loose on coastal ones, and the size of the error depends on the leg. The figures below come from comparing the network with 60 published distances: 45 port pairs from NGA Pub. 151, Distances Between Ports (11th edition, 2001), and 15 from NOAA’s Distances Between United States Ports (15th edition, 2026), both United States government works, with each port at the table’s own position.

The network runs about 1% long on ocean routes. Pub. 151 measures between “central positions that most represent each port”, NOAA between “a centrally located pier inside the port”, and the two differ from each other by a few miles at the same port for that reason alone. The worst coastal case, Al Jubayl to Ras Tanura, is 34 nautical miles in Pub. 151 and 149 on the network, which joins neither port near the coast. Mobile to New Orleans reads 37% short, because the network cuts across Breton Sound where NOAA measures by Southwest Pass.

Canal and strait distances

The three canals the calculator can close are measured at their lane length, and the official figures are these.

The Suez Canal Authority publishes its own comparison of the canal route with the Cape of Good Hope. Singapore to Rotterdam is 8,288 nautical miles through the canal against 11,755 around the Cape, and Jeddah to Piraeus is 1,316 against 11,207. Switching off the Suez Canal in the calculator gives the Cape route for any pair, and the result box gives the extra days at the speed entered.

Emission control areas on the route

An emission control area (ECA) is a sea area where MARPOL Annex VI Regulation 14.4 limits the sulphur in fuel oil to 0.10% m/m, and the calculator counts a mile as an ECA mile only when that limit applies on the sailing date. Regulation 14.7 exempts ships from the limit for the first 12 months after an amendment designating an area enters into force, so an area can exist in law and still not count.

The boundaries come from the coordinates in each resolution, joined by geodesic lines where the text says so; the Baltic, North Sea and Norwegian Sea are defined by the parallels, meridians and points of the Annex I, V and II definitions. Where a boundary is “the coast”, the calculator’s polygon is closed over land, which a route never crosses. The Mediterranean area excludes the box at the Suez Canal’s northern entrance between 31°14’ N and 31°29’ N that MEPC.361(79) sets, which holds Port Said.

The North-East Atlantic area adopted at MEPC 84 in 2026 is not counted. IMO gives its entry into force as 1 September 2027 and its effect as 12 months later, and lists resolution MEPC.407(84), which carries its coordinates, as awaiting certification. The Canadian Arctic polygon follows the outer line of MEPC.392(82) from coast to coast, so Hudson Bay and Ungava Bay are inside it.

Rotterdam to Singapore on 2 March 2026 runs 421.0 nautical miles in the North Sea area and 1,941.4 in the Mediterranean, 28% of the 8,411.0. Houston to Rotterdam runs 940.0 in the North American area and 399.9 in the North Sea area, 26.6% of 5,045.7.

Where each input comes from

• Ports. The port list is the World Port Index, which names many ports in the local form: Piraievs for Piraeus, Jiddah for Jeddah, Bur Sa’id for Port Said. The English name finds the port in either form. Each port is shown with its country, and where one country has two ports of one name, with the state from its UN/LOCODE entry: the list holds two Portlands in the United States, Portland, OR on the Columbia River and Portland, ME on Casco Bay. A name typed without the state finds the larger port, and the sentence under the inputs names the port that was picked.
• Speed. The service speed from the charter party, or the speed the voyage instructions set. For a passage with a known current, use the speed over the ground, as in the ship ETA calculator.
• Sailing date. The departure from the berth or the pilot station, in UTC. The date only changes which emission control areas count.

About the data

The lane network is Eurostat’s SeaRoute MARNET, licensed under the European Union Public Licence 1.2; the copy this calculator loads was converted to a compact binary on 26 October 2026 and is published with its license at /data/sea-distance/. Port positions are from the NGA World Port Index and Pub. 151, works of the United States government, and from Wikidata, whose data is in the public domain. The coastline used to keep port legs at sea is Natural Earth’s, also in the public domain.