Calculate buoyant force, displacement, and stability characteristics based on hull dimensions.
Typical displacement cruiser
Lightweight planing hull
Wide, stable multihull
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Download NowBuoyancy is the upward force water exerts on a hull, and it is the reason a boat floats at all. By Archimedes' principle that force equals the weight of the water the hull pushes aside, so a boat sinks into the water until the displaced weight matches its own, and no further.
For a builder, buoyancy answers two practical questions: how much weight the hull can carry before it reaches its scuppers, and how much reserve flotation sits above the load waterline to keep the boat dry and stable when loaded or swamped.
Enter length, beam and draft, pick a hull form, and the calculator returns the displaced volume, the buoyant force, the equivalent displacement tonnage, an estimate of reserve buoyancy and a metacentric-height figure to sanity-check the design.
Buoyant Force (lb) = Displaced Volume (ft³) × Water Density; Displacement (tons) = Force ÷ 2,240| Draft (ft) | Volume (ft³) | Force Fresh (lb) | Force Salt (lb) |
|---|---|---|---|
| 1 | 160 | 9,984 | 10,240 |
| 2 | 320 | 19,968 | 20,480 |
| 3 | 480 | 29,952 | 30,720 |
| 4 | 640 | 39,936 | 40,960 |
Approximate buoyant force per foot of draft on an 8 ft beam hull.
Formula verified against standard naval-architecture references · Updated October 2026
Buoyancy is the upward push a fluid gives to anything placed in it. Water presses up on a submerged hull with a force equal to the weight of the water the hull displaces, which is Archimedes' principle. A boat floats when that upward buoyant force balances the downward weight of the boat and its load. Add more weight and the boat sinks deeper until it displaces enough extra water to match.
The maximum load equals the buoyant force at your deepest acceptable draft minus the boat's own weight. In practice you size so the fully loaded boat still shows designed freeboard, not the last inch of reserve. This calculator gives the buoyant force for your draft; subtract hull, engine and fixed gear to find the useful payload with a safe margin.
Reserve buoyancy is the watertight volume of the hull above the loaded waterline. It is the extra flotation that comes into play when the boat heels, pitches, takes water aboard or is slightly overloaded, and it is a major contributor to safety in a seaway. Full-bowed runabouts and trawlers with big flared bows carry a lot; low-freeboard skiffs carry little, so every inch of reserve matters more.
Yes, because salt water is denser. One cubic foot of seawater weighs about 64.0 lb versus 62.4 lb in freshwater, so a boat displaces slightly less volume to float the same weight and rides a touch higher in the ocean. Cross from sea to a large lake and you will see the load waterline drop by a small but real amount.
Add sealed, water-immune buoyancy that exceeds the boat's own weight even when swamped. Closed-cell foam in the hull, sealed tanks under thwarts and bulkhead compartments all work. Size the flotation so the swamped hull and motor still float with the gunwales clear, following the Coast Guard or CE formula for your boat's weight and the occupant load.
Hull shape governs how quickly buoyant force builds as the boat settles. A broad, boxy pontoon gains a lot of force per inch of draft, so it carries heavy loads on shallow draft. A fine, v-shaped hull gains force slowly, so it needs more depth for the same weight, but it cuts through waves more gently. Choose the form that matches load, draft and sea state.