Draft Survey Calculation Explained: The Complete Step-by-Step Method (2026)

A proper draft survey reads the draft at forward, midship and aft, on both port and starboard sides — six readings, not three. Reading only one side and assuming symmetry is a common shortcut that introduces error before the calculation has even started, particularly on a ship with any degree of list.

Each pair (STBD and PORT at the same position) is averaged into a mean draft for that position. This is the raw data every subsequent step is built on, and it is worth the extra few minutes at the rail to get it right — no formula downstream corrects for a misread draft mark.

Draft marks are rarely painted exactly at the forward and aft perpendiculars — there is almost always a small offset, measured in metres, that must be corrected for using the ship's own particulars and the observed trim. Skipping this correction is a small error individually, but on a ship with a significant offset it compounds directly into the displacement lookup in Step 4.

A ship's hull is not perfectly rigid: under certain loading conditions it flexes very slightly, hogging (bowing upward at midship) or sagging (bowing downward at midship). The corrected midship draft is compared against the mean of the corrected forward and aft drafts — if the midship reading is lower than that mean, the ship is hogging; if higher, sagging.

This is not a rounding-error correction. On a fully loaded bulk carrier it can shift the calculated draft by several centimetres, which at typical bulk carrier tonnes-per-centimetre figures translates directly into tens of tonnes of displacement. Any draft survey explanation that jumps straight from corrected drafts to a mean draft without addressing hog and sag is describing an incomplete method.

The corrected drafts are combined into the quarter mean draft — weighted six parts midship to one part each forward and aft — which is the single figure used to enter the ship's hydrostatic table. From it, displacement, TPC (tonnes per centimetre immersion), MTC (moment to change trim) and LCF (longitudinal centre of flotation) are read or interpolated.

This is also where hand calculation becomes genuinely error-prone: a real hydrostatic table runs to hundreds of rows in fine draft increments, and the quarter mean draft rarely lands exactly on a tabulated value. Interpolating correctly between two rows, under time pressure, with a cargo operation waiting, is the single step most likely to introduce a transcription error into an otherwise sound survey.

Displacement read straight from the quarter mean draft assumes the ship is on an even keel, which it almost never is. Two further corrections adjust for trim: a first trim correction using TPC, LCF and the trim itself, and a second trim correction using the rate of change of MTC with draft — a refinement that matters more on ships with a longer LBP and larger trim angles.

Both corrections are applied to the same reference length, the ship's LBP — a detail that is easy to get wrong by substituting the length between draft marks instead, which produces a plausible-looking but incorrect result.

Displacement tables are built for a reference water density, usually 1.025 t/m³. The actual dock water density, measured on site, is almost always different, and the trim-corrected displacement must be adjusted proportionally to reflect the water the ship is actually floating in.

The result is the corrected displacement — repeated for both the initial survey (before loading or discharging) and the final survey (after). Subtracting known deductible weights (bunkers, fresh water, ballast, other consumables) from each gives the net displacement at each end, and the difference between the two net displacement figures is the cargo quantity: the number that goes on the mate's receipt and gets checked against the bill of lading.

In practice, disputes rarely trace back to a single dramatic error. They trace back to the accumulation of several small ones: a hydrostatic table interpolation off by one row, a trim correction calculated against the wrong reference length, a hog/sag correction skipped entirely under time pressure. Each is individually small. Together, on a large parcel, they can shift the calculated cargo quantity by an amount worth arguing about.

The fix is not a different method — the quadratic mean draft survey is the industry standard for a reason — it is removing the manual steps where transcription and interpolation errors actually happen.

Draft Survey Calculator Pro on ShipCrewFinder runs this exact method — hog/sag correction, quarter mean, first and second trim correction, density correction — against your own vessel's hydrostatic table, entered once and reused for every future survey. Every intermediate figure is shown, not hidden, so the calculation can be checked line by line, and the final report exports as a clean, signable PDF.

The formula chain was built and verified against a real vessel's certified draft survey worksheet, matched intermediate value by intermediate value to the last decimal — the same standard any surveyor would hold a manual calculation to.