Vessel Displacement Calculator
Calculate vessel displacement from hull dimensions, draft, and block coefficient.
About this calculator
A floating vessel displaces a volume of water whose weight exactly equals the vessel's own weight — this is Archimedes' principle, and it's the basis of every displacement calculation. This calculator builds that volume from three hull dimensions (waterline length, beam, and draft) and a block coefficient, which is the ratio of the underwater hull's actual volume to the volume of the rectangular box that would just enclose it. A block coefficient of 1.0 would mean a hull shaped like a barge with square ends; real hulls range from roughly 0.35–0.55 for fine, fast sailboat and yacht hulls to 0.8–0.9 for boxy, capacity-optimized cargo ships.
Multiplying length x beam x draft x block coefficient gives underwater volume, and multiplying volume by water density gives displacement weight — saltwater's 64 lb/ft³ versus freshwater's 62.4 lb/ft³ means this calculator's fixed underwater volume (set by the length, beam, draft, and block coefficient you enter) displaces slightly more weight in saltwater than the identical volume would in freshwater. That's a different framing from asking what happens to a boat of fixed weight: hold the vessel's weight constant instead, and moving it from saltwater to freshwater means it needs to sit slightly deeper — displacing more volume — to support that same weight in the less-dense water. What this tool does not account for: trim (fore-aft tilt) or heel (side-to-side tilt) at speed, which change the actual wetted hull shape from the static case; hull growth from fouling; or load distribution — displacement gives you total weight capacity, not where that weight should sit for proper trim.
Inputs
Results
Displacement (Long Tons)
33 LT
How to Use This Calculator
- Enter the waterline length, beam, and draft of the vessel in feet.
- Input the block coefficient (Cb) — typical values: ~0.8-0.9 for cargo ships, ~0.35-0.55 for sailboats and yachts.
- Select water type (saltwater = 64 lb/ft³, freshwater = 62.4 lb/ft³).
- Review displacement in long tons and metric tons for loading and stability calculations.
- Compare the underwater volume in ft³ and m³ for buoyancy verification.
How the result changes with Waterline Length
| Waterline Length | Displacement (Long Tons) |
|---|---|
| 18 | 16.97 LT |
| 26 | 24.51 LT |
| 53 | 49.97 LT |
| 88 | 82.97 LT |
What each input means
- Waterline Length
- Length at the waterline, not overall length.
- Beam (Width)
- Maximum width of the hull at the waterline.
- Draft
- Depth of hull below the waterline.
- Block Coefficient (Cb)
- Ratio of underwater volume to enclosing box. Sailboats ~0.35-0.55, cargo ships ~0.8-0.9.
- Water Type
- Saltwater is denser, resulting in slightly more buoyancy.
How this is calculated
Worked example, using the default values
- Identify Input Parameters5 parametersWaterline Length = 35, Beam (Width) = 12, Draft = 5, Block Coefficient (Cb) = 0.55, Water Type = Saltwater = 5 input(s) provided
- Calculate DisplacementDisplacement33 = 33
- Calculate DisplacementDisplacement33.53 = 33.53
- Calculate DisplacementDisplacement73920 = 73920
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the same hull displace less weight in freshwater than saltwater?
Displacement weight equals underwater volume multiplied by water density, and saltwater is denser (about 64 lb/ft³) than freshwater (about 62.4 lb/ft³) due to dissolved salts. For a hull of fixed shape floating at a given draft, that means the identical underwater volume weighs more in saltwater — equivalently, a boat moved from saltwater to freshwater will actually sit slightly deeper to displace the extra volume needed to support the same weight.
What does block coefficient actually measure?
Block coefficient (Cb) is the ratio of a hull's actual underwater volume to the volume of a rectangular box with the same length, beam, and draft. A low Cb around 0.35 describes a fine, tapered hull shape typical of fast sailboats and racing yachts, while a high Cb near 0.9 describes a boxy, full-bodied hull typical of cargo ships — which sacrifice speed for maximum cargo-carrying volume per unit of length.
Does increasing beam or draft change displacement by the same amount as length?
All three dimensions increase displacement volume when increased, since the formula multiplies length, beam, draft, and block coefficient together — but the proportional effect of any one dimension depends on the vessel's actual dimensions, because each is one factor in a product with the other two. A wider or deeper hull at the same length and block coefficient does displace more, in direct proportion to how much that dimension itself grew.
Is displacement the same thing as a boat's dry weight?
Not quite — displacement is the total weight the hull is currently supporting while floating at the given draft, which includes the boat's structure, engines, fuel, water, gear, and crew aboard at that moment. A vessel's dry or 'light ship' weight is lower than its loaded displacement; naval architects typically calculate displacement at several load conditions (light, half-load, full-load) rather than treating it as one fixed number.
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