Draft Survey Calculator
Compute cargo quantity by draft survey with mean-of-means, trim and density corrections, deductibles, and a B/L quantity tolerance check.
Overview & methodology
How draft survey cargo quantity is determined
A draft survey establishes cargo quantity by comparing the vessel's net displacement before and after loading or discharge. Six draft readings (forward, midships and aft, port and starboard) are averaged into a mean-of-means using the quarter-mean formula, which corrects for hull deflection. Displacement is then read from the hydrostatic tables and corrected for trim (first and second trim corrections) and dock-water density before deducting ballast, fresh water, fuel, lube oil, sludge and the vessel's constant.
The difference between the initial and final net displacements is the cargo loaded or discharged. Comparing this figure against the bill of lading quantity flags discrepancies outside the customary ±0.5% survey tolerance, which is important for claims and demurrage/despatch calculations.
Frequently asked questions
How is cargo quantity calculated from a draft survey?
Take the six drafts, compute the mean of means, apply trim (first and second) and density corrections to displacement, then subtract deductibles (ballast, fresh water, fuel, stores, constants). Cargo = final displacement on completion − final displacement on arrival.
What tolerance applies between the draft survey and the B/L figure?
Commercially, a draft survey is accepted to about ±0.5% against the bill of lading quantity. Larger differences normally trigger a re-survey, a letter of protest, or a shore-scale reconciliation before the master signs.