Draft Survey Calculator: Bulk Cargo Weight for Settlement
Calculate bulk cargo weight for invoice settlement using draft survey methodology. Model vessel displacement, ballast, and constant adjustments.
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Overview & methodology
What is a draft survey?
Draft Survey Formulas
Quarter Mean Draft = (Df + Da + 6 * Dm) / 8First Trim Correction = Trim * LCF * TPC / LBPSecond Trim Correction = Trim^2 * (MTC+50 - MTC-50) * 50 / LBPDensity Correction = Displacement * (Density(dock) - 1.025) / 1.025Net Cargo = Final Net Displacement - Initial Net DisplacementDf = mean of forward port/stbd drafts, Da = mean aft drafts, Dm = mean midships drafts. Deductibles (ballast, fuel, fresh water, constant) are removed from each gross displacement before taking the difference.
The 6 UN ECE Standard Draft Survey Steps
| Step | Name | Purpose |
|---|---|---|
| 1 | Apparent Drafts & Stem/Stern Corrections | Read forward, midships and aft drafts at both port and starboard, then correct for the position of the draft marks relative to the perpendiculars. |
| 2 | Quarter Mean Draft (Nemoto's Formula) | Combine the six corrected readings into a single mean draft using the weighted quarter-mean (Nemoto) formula. |
| 3 | First Trim Correction | Adjust displacement for the vessel's trim relative to the LCF (longitudinal centre of flotation). |
| 4 | Second Trim Correction | Apply a further correction for the rate of change of MTC (moment to change trim) with draft. |
| 5 | Dock Water Density Correction | Adjust displacement for actual dock water density versus the 1.025 t/m3 reference density in the hydrostatic tables. |
| 6 | Deductibles Survey | Sound and record ballast, fuel, fresh water, lube oil and the vessel's constant, then subtract from gross displacement for net cargo weight. |
Numerical Example: Cargo Quantity Determination
These are the calculator's default readings. Net displacement is the hydrostatic displacement after trim and density corrections, minus ballast, fuel, fresh water and the ship's constant. The 0.33% gap sits within the customary 0.5% survey tolerance.
Related tools: Stowage Factor, Port Drafts.
Method note: calculations run in your browser using the formulas and published rule books cited on this page. Results are free estimates for planning, with no login required, and should be confirmed with your bank, broker or the official source before you commit.
Frequently asked questions
What is a draft survey?
A draft survey determines the weight of cargo loaded or discharged by reading a vessel's draught marks before and after cargo operations and applying hydrostatic corrections for ballast, fresh water, fuel and other deductibles. It is the standard method for settling bulk cargo quantities when shore scales are unavailable.
Why do I need density and trim corrections?
The hydrostatic tables are based on the vessel floating in water of a specific density (usually 1.025) and on even keel. Actual water density and trim change the displacement, so surveyors correct the mean draught and apply TPC and MTC corrections before reading displacement.
What is the 'constant' in a draft survey?
The constant is the difference between the vessel's lightship weight and the actual weight of everything on board besides cargo: ballast, bunkers, fresh water, lubricating oil, sludge, stores and crew effects. It is established at the initial survey and applied to the final survey.
How accurate is a draft survey?
A well-conducted draft survey is typically accurate to within 0.5% of cargo weight. Accuracy depends on the quality of draught readings, a complete ballast survey, correct hydrostatic data and calm sea conditions.
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