Port Dues Calculator: Gross Tonnage × Tariff Rate

Formula, assumptions, and limits

To calculate port dues, multiply the gross tonnage by the tariff rate on the basis the tariff states, then by the number of calls:

$$D = GT \cdot r \cdot k$$

GT - gross tonnage per the ship’s ITC 1969 tonnage certificate. r - the published rate; a per-100-GT rate divides by 100 before the multiplication. k - calls, or tariff periods where dues are time-bounded.

The linear core is what nearly every tariff shares; the minimums, caps, bands, duration elements, and rebates that decorate it are covered in their own section below, and they modify this calculator’s output rather than replace it.

GT is a dimensionless index of enclosed volume under the International Convention on Tonnage Measurement of Ships, 1969, not a weight and not deadweight. The tonnage certificate’s GT is the figure the tariff means, and the same hull keeps it for life barring conversion.

How to use this calculator

1. Enter the gross tonnage from the tonnage certificate.
2. Enter the rate from the port authority’s published tariff, in the tariff’s currency.
3. Select the basis the tariff states: per GT, or per 100 GT.
4. Enter the number of calls the estimate covers.
5. Read the dues per call and the period total; the curve shows how the line scales with GT, useful when comparing candidate ships for a trade.

Worked example

A 40,000 GT bulk carrier calling at a port whose tariff states 0.45 per GT per call (currency as published). Dues: 40,000 × 0.45 = 18,000 per call; a service making 12 calls a year carries 216,000 on this line alone. The same tariff stated as 45 per 100 GT is the identical figure: basis matching, not arithmetic, is where estimates go wrong. The rate is illustrative; real rates live in the port’s current tariff schedule, and the two cited tariff pages (Rotterdam, Singapore) show the published form the input should come from.

Common errors

1. Deadweight in the GT field. Tariffs reference gross tonnage; a Handysize’s 35,000 dwt against its roughly 22,000 GT overstates the line by more than half.
2. Basis mismatch. A per-100-GT rate entered as per GT inflates the result a hundredfold; the tariff’s own wording settles it.
3. Ignoring minimums and caps. The linear figure undershoots small craft (the minimum applies) and can overshoot ULCs and cruise ships where the tariff caps the line.
4. Missing the rebate. Frequent-caller and environmental-rating discounts are claimed, not automatic; a service that qualifies and does not claim pays the full line all year.

Where each input comes from

• Gross tonnage: the International Tonnage Certificate (1969) in the ship’s document file; the figure also appears in class records and the major registries. Not the deadweight from the chartering description.
• Rate and basis: the port authority’s current tariff schedule, published on its website (the Rotterdam and Singapore pages cited below are the form to look for). Take the rate for the ship’s type and trade where the tariff distinguishes them.
• Calls: the service schedule for liner trades; one, for a tramp estimate.

Tariff structures beyond the linear rate

The per-GT rate is the core, but reading a tariff means recognizing the other shapes. Banded tariffs step the rate by size bracket, so a ship near a band edge pays a visibly different rate per GT than one slightly smaller. Duration elements re-incur dues when a stay exceeds the covered period, which matters for ships working slow cargoes or under repair afloat. Laid-up tariffs charge a reduced rate for idle tonnage. And the rebate layer has grown: frequent-caller schemes return a percentage to liner services, while environmental schemes discount dues for ships holding recognized emissions ratings, a lever port authorities increasingly use as decarbonization policy. None of these change the arithmetic this page does; they change which rate and how many periods you feed it.

Naming deserves a caution of its own. “Port dues”, “harbour dues”, “tonnage dues”, and “marine dues” travel badly between jurisdictions: in some tariffs they are synonyms, in others the port authority’s GT-based call charge sits beside a separate national levy against net tonnage for waterway or light services. Read the tariff’s own definitions before mapping its lines onto this calculator, and run each separately tariffed line as its own calculation.

When to use this calculator

Use this page when the port dues line itself is the question: tariff arithmetic for a voyage estimate, a fleet comparison across candidate ships, or a check on a proforma’s largest tonnage-driven line. When the whole call’s cost is the question, the port disbursement account estimator carries this line alongside pilotage, towage, mooring, and the agency fee; this calculator feeds that one’s port-dues row.

About This Port Dues Calculator

Port dues are the tonnage-driven core of every disbursement account: the port authority’s infrastructure charge on the ship, tariffed against gross tonnage in most of the world’s ports. This calculator is for voyage estimators, operators, and agents who need the line computed from the tariff: it takes the GT, the published rate on its stated basis (per GT or per 100 GT), and the call count, and returns the per-call and period figures.

The arithmetic is deliberately the tariff’s own: a linear rate against certificate GT, with the tariff’s minimums, caps, bands, and rebates applied by the user where they bite, because those vary by port and tariff year. The cited Rotterdam and Singapore tariff pages show the published form the rate input should be taken from. The GT sensitivity curve draws the line across vessel sizes around your entry, which turns the calculator into a comparison tool: two candidate ships for one trade differ on this line by exactly the tariff slope.

The curve ships with the same accessibility contract as every chart on this site: a data table fallback and keyboard-reachable points. The per-call figure drops straight into the disbursement account estimator as its first row.

Further reading

• Port dues
• Disbursement account
• Port disbursement account estimator
• Tonnage measurement

Frequently asked questions

What are port dues?

Port dues are the port authority's charge on the ship for entering and using the port: the general infrastructure charge, distinct from service charges like pilotage and towage and from terminal charges on the cargo. Most ports tariff them against the vessel's gross tonnage.

How are port dues calculated?

Multiply the ship's gross tonnage by the tariff rate, matching the basis the tariff states: a rate quoted per 100 GT divides by 100 first. Many tariffs add minimums, size bands, or caps, and rebate schemes for frequent callers or green tonnage modify the linear figure.

Is GT the same as deadweight?

No. Gross tonnage is a dimensionless index of the ship's enclosed volume under the ITC 1969 convention; deadweight is the carrying capacity in tonnes. Port tariffs reference GT from the tonnage certificate, and confusing the two is the classic port-dues estimating error.

Who pays port dues?

The shipowner or operator pays the port, through the appointed agent's disbursement account. Under most time-charter forms the cost allocation shifts to the time charterer as a trading expense, while under voyage charter it stays with the owner inside the freight.

What is the difference between port dues and tonnage dues?

Usage varies by country, but where the terms are distinguished, port (or harbour) dues are the port authority's GT-based charge for the call, while tonnage or marine dues are levied by a national authority, sometimes against net tonnage, for waterway or light services. Read the tariff's own definitions; the names travel badly between jurisdictions.

Do all ports charge dues the same way?

No. Per-GT and per-100-GT linear rates are the common bases, but ports also use banded steps, per-call minimums and maximums, laytime-style duration elements, and discounts by ship type, frequency, or environmental rating. The published tariff governs; this calculator handles the linear core.