How This Calculator Works

This calculator follows the standard Quarter Mean (QM) method used by marine surveyors worldwide, as described in the UNECE Code of Uniform Standards for Draught Surveys:

Quarter Mean Draft — Averages six draft readings (Fwd Port/Stbd, Mid Port/Stbd, Aft Port/Stbd), with the midship reading weighted 6× to suppress hull deflection (hogging/sagging).
Trim Correction (1st & 2nd) — Corrects for the fact that the LCF (longitudinal centre of flotation) is not at amidships.
Density Correction — Converts displacement from the hydrostatic table's standard density (usually 1.025 t/m³ for seawater) to actual measured harbour water density.
Deductibles & Cargo Weight — Subtracts ballast water, fuel, fresh water, and constant to arrive at net cargo loaded or discharged.

Input Data

Draft Readings (metres)

Forward Port (m)

Forward Stbd (m)

Midship Port (m)

Midship Stbd (m)

Aft Port (m)

Aft Stbd (m)

Water Density

Measured Density (t/m³)

Ship Data (from Hydrostatic Tables)

LBP (m)

LCF (m from midship, + aft)

TPC (tonnes/cm)

MTC (t-m/cm, optional)

Additional Weights (tonnes)

Ballast Water (t)

Fresh Water (t)

Fuel Oil (t)

Diesel Oil (t)

Lubricating Oil (t)

Ship's Constant (t)

Calculation Results

Enter values and click Calculate to see step-by-step results.

Related Resources

Draft Survey Calculation Guide
Full formulas, worked examples, and common errors
How to Read Draft Marks
Practical guide to accurate draft reading in port conditions

Did You Know?

On a Capesize bulk carrier, a draft reading error of just 1 centimetre represents approximately 60-100 tonnes of cargo. That is why the Quarter Mean formula is essential -- it weights the midship reading six times more than forward and aft, suppressing the effect of hull bending (hogging or sagging).

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