Draft Survey Cargo Calculator
A draft survey weighs bulk cargo by the water a ship displaces. You read the ship's displacement from its hydrostatic tables before and after loading, then strip out everything that is not cargo: ballast, fuel, fresh water, and the ship constant. The displacement difference, less the change in those deductibles, is the cargo loaded or discharged. Enter the light and loaded displacements together with the deductible totals at each survey and this calculator returns the net cargo weight and the gross displacement change.
Draft survey formula
Gross change = loaded displacement - initial displacement
Deductible change = final deductibles - initial deductibles
Net cargo = gross change - deductible change
Cargo vs gross = net cargo / gross change x 100
Displacement is read from the ship's hydrostatic tables at the corrected mean draft for each survey. Deductibles cover ballast, fuel, fresh water, oils, and the constant. The remainder after removing the deductible change is the cargo handled.
Draft survey notes
- Displacement must be corrected for water density and trim before use; take those figures from the ship's tables.
- Sound all tanks carefully; ballast and fuel errors map directly into cargo error.
- The ship constant is held the same in both the initial and final deductibles.
- Calm water and a steady ship give the most reliable readings.
- A good survey reaches about 0.5 percent accuracy on the cargo figure.
Draft survey: frequently asked questions
What is a draft survey?
A draft survey is a method of weighing bulk cargo by measuring how much deeper a ship sits in the water after loading. The displacement before loading is subtracted from the displacement after loading, and known weights like ballast water, fuel, and fresh water are deducted to leave the net cargo weight.
How is cargo weight calculated in a draft survey?
Net cargo equals the loaded displacement minus the light (initial) displacement minus the change in deductibles. In its simplest form: cargo = (loaded displacement - light displacement) - (final deductibles - initial deductibles). Deductibles include ballast, fresh water, fuel oil, and constant.
What are deductibles in a draft survey?
Deductibles are everything aboard that is not cargo: ballast water, fuel oil, diesel oil, fresh water, lubricating oil, and the ship constant. Their combined weight is measured or estimated before and after loading so it can be removed from the displacement difference, leaving only the cargo.
What is the ship's constant?
The constant is the difference between the displacement read from the hydrostatic tables at light condition and the actual known light weights aboard. It accounts for items not otherwise measured, such as mud in tanks, stores, and spare parts. It is treated as a fixed deductible across the survey.
How accurate is a draft survey?
A careful draft survey on a steady ship in calm water can determine cargo weight to within about 0.5 percent. Accuracy depends on calm conditions, accurate draft reading, correct density correction, and reliable tank soundings. This calculator does the displacement arithmetic; density and trim corrections come from the ship's tables.
Official sources
- International Maritime Organization: IMO.
- NOAA Office of Coast Survey: Nautical Charts.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.