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Draft survey calculation step by step: from draft marks to cargo weight

A draft survey works out the weight of cargo from the ship’s drafts before and after loading or discharging. The method is the same on every survey sheet. Below are the steps in order, with the formula for each, so you can follow — and check — a surveyor’s figures.

Updated 7 October 2026 · Written by SeaReady in our own words. Not affiliated with OCIMF, RightShip, CDI, any MoU or any administration — always check the current official documents.

The steps

  1. Read six drafts: forward, midships and aft, port and starboard. Take the mean of each pair.
  2. Correct to the perpendiculars: the marks are not at the forward and aft perpendiculars or exactly midships. Using the observed trim over the distance between the marks, correct each mean draft to F (forward perpendicular), M (midships) and A (aft perpendicular).
  3. True trim: A − F (positive = by the stern).
  4. Quarter mean: (F + 6M + A) ÷ 8. Round to the millimetre as surveyors do.
  5. Hydrostatics: from two table rows that bracket the quarter mean, interpolate displacement, TPC and LCF.
  6. First trim correction: trim × LCF (from midships) × TPC × 100 ÷ LBP. It is positive when the LCF is on the same side of midships as the deeper end (aft LCF with the ship trimmed by the stern), negative otherwise.
  7. Second trim correction: 50 × trim² × (MTC at quarter mean + 0.5 m − MTC at quarter mean − 0.5 m) ÷ LBP.
  8. Density correction: displacement × dock-water density ÷ table density (usually 1.025).
  9. Deductibles: subtract ballast, fresh water, fuel, lube oil and other weights on board.
  10. Cargo: the difference between the initial and final surveys — or net displacement minus lightship and the ship’s constant.

Quick reference

Method: UN/ECE Code of Uniform Standards and Procedures for the Performance of Draught Surveys of Coal Cargoes (ECE/ENERGY/19, 1992). Second trim correction as in Wawrzyński, TransNav 18(3), 2024.
ItemFormula
Quarter mean(F + 6M + A) ÷ 8
Hog / sagM − (F + A) ÷ 2 (positive = sagging)
1st trim correction (t)trim × LCF × TPC × 100 ÷ LBP
2nd trim correction (t)50 × trim² × ΔMTC ÷ LBP
Density correctionΔ × ρ dock ÷ ρ table

Where draft surveys go wrong

  • Reading drafts in a swell without averaging over several waves, or from the wrong side only.
  • Mark distances taken from the wrong reference, or with the wrong sign — note whether each mark is forward or aft of its perpendicular.
  • LCF read with the wrong sign: some booklets give it from midships (forward or aft), others from the aft perpendicular.
  • Dock-water density sampled at one depth only, or with a dirty hydrometer.
  • Soundings of ballast and fresh water taken at a different time from the drafts, or tank tables used without trim correction.

Loading records and stability calculations are checked in RightShip inspections and in management-level orals.

Questions seafarers ask

What is the quarter mean draft?

The quarter mean (also called the mean of mean of means) is (F + 6M + A) ÷ 8, using drafts already corrected to the forward perpendicular, midships and the aft perpendicular. It allows for the hull hogging or sagging, so the displacement taken from the hydrostatic table is closer to the truth than a simple mean.

Why are there two trim corrections?

The hydrostatic table assumes the ship is on an even keel. The first trim correction moves the displacement to the true centre of flotation (it depends on trim, LCF, TPC and LBP). The second (Nemoto) correction allows for the LCF moving as the ship trims: 50 × trim² × (MTC at quarter mean +0.5 m − MTC at −0.5 m) ÷ LBP. It is always positive.

How do I correct for dock water density?

Multiply the displacement corrected for trim by the measured dock-water density and divide by the density the hydrostatic table is made for — usually 1.025 t/m³.

Can a draft survey calculator replace the loading computer?

No. A calculator like this is a cross-check. Use your ship’s class-approved loading computer and stability booklet (SOLAS II-1/5-1, IACS UR L5), and check every figure against the booklet.

Run a draft survey cross-check

SeaReady’s draft survey calculator takes the six draft readings, mark distances and two hydrostatic rows, and shows every step with its formula. It works offline. Cross-check only — never a loading instrument.

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