The chartering question on every voyage is simple: how much cargo can this ship actually lift? This calculator starts from deadweight at the sailing draft and deducts bunkers, fresh water, ballast, and the vessel constant to give the maximum cargo intake and the percentage of deadweight that cargo represents.
How it works
Cargo is whatever deadweight remains after the non-cargo weights are removed:
deductibles = fuel oil + diesel oil + fresh water + ballast + constant
cargo intake = deadweight − deductibles
utilisation% = cargo intake / deadweight × 100Deadweight itself is fixed by the displacement at the permitted draft, so on a draft-restricted port the cargo you can lift is whatever is left after the required bunkers and constant are subtracted from that limited deadweight.
Worked example
A Panamax bulk carrier has 72,000 DWT at her summer load line, but is constrained to a 12.5 m sailing draft on this voyage, giving 69,500 DWT at that draft. Deductibles: 1,800 t fuel oil, 150 t diesel, 280 t fresh water, 0 t ballast retained, and a 300 t vessel constant — totalling 2,530 t. Cargo intake = 69,500 − 2,530 = 66,970 t, at 96.4% utilisation of the draft-limited deadweight.
If the voyage were longer and required 3,200 t of bunkers instead of 1,800 t, intake would fall to 65,570 t and utilisation to 94.3%. The charter party’s guaranteed intake clause, if any, must be checked against this figure.
Key concepts for operators
The vessel constant explained
The constant is not a stable number — it drifts upward over a ship’s life as mud, scale, and unaccounted stores accumulate. A thorough constant survey (comparing calculated lightship weight from hydrostatics against an actual empty-vessel displacement) periodically re-establishes the true figure. Using an outdated or underestimated constant can cause a vessel to load beyond her marks without the operator realising it.
Why utilisation percentage matters commercially
DWT utilisation directly drives voyage profitability. Freight is earned on cargo tonnes; bunkers, port costs, and canal dues are largely fixed per voyage regardless of cargo weight. A voyage at 90% utilisation versus 96% on a 70,000-DWT vessel is a difference of roughly 4,200 t of cargo. At any freight rate above zero, that lost cargo represents real revenue that cannot be recovered once the vessel sails.
Bunker planning and the cargo/bunker trade-off
For long voyages, operators must balance bunker quantity against cargo intake. Carrying more bunkers reduces cargo intake but provides safety margin and may allow slower steaming (slow steaming burns less fuel per day, but a longer voyage consumes more total days). For a voyage calculation to be useful, the bunker figure entered should reflect the departure quantity, not the arrival quantity — the full amount on board when leaving the load port.
Draft restrictions at multiple ports
If a voyage involves several ports with different draft limits (for example, a shallow load port followed by a deeper discharge port), the calculation must be run at the most restrictive draft on the route. The draft-limited deadweight at the shallowest port sets the cargo ceiling for the entire voyage, even if the ship could legally carry more on arrival.
Practical tips
- Always source DWT from the vessel’s current hydrostatic tables at the relevant draft, not the design summer DWT on the classification certificate.
- Obtain the vessel constant from the last verified constant survey, and add a small margin if the survey is older than 12 months.
- For tanker voyages, include heating coil steam and inert gas system consumables in the constant or as a separate deductible where material.
- Cross-check the intake figure against the cargo hold cubic capacity — a high-density cargo may hit the weight limit before the holds are full, while a low-density bulk may fill the holds before the weight limit is reached.