Calculate displacement and buoyancy for your boat hull
Typical small cruising sailboat
Mid-size motor yacht
High-performance racing dinghy
feet
feet
feet
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Download NowHull volume tells you how much water your boat's underwater shape displaces, and from that how much it can weigh and still float. For a DIY builder it is the first reality check on a design: get the volume wrong and every downstream number, from displacement to ballast to freeboard, is wrong too.
Because a hull is not a neat rectangular box, we multiply the length, beam and draft by a block coefficient that captures how full or fine the underwater form is. A blunt, cargo-carrying barge sits near the top of the range; a fine-lined sailboat sits near the bottom. Using a typical value gives a close estimate without lofting every section.
This tool takes your length, beam and draft in feet, applies a standard displacement-hull block coefficient, and returns the displaced volume in cubic feet along with the weight of water displaced and a few useful form ratios.
Displaced Volume (ft³) = L × B × T × Cb, then Buoyant Force (lb) = Volume × 64.4| L × B × T (ft) | Volume (ft³) | Buoyant Force (lb) |
|---|---|---|
| 12 × 4 × 1.5 | 32 | 2,090 |
| 16 × 6 × 2 | 86 | 5,570 |
| 20 × 7 × 3 | 189 | 12,170 |
| 26 × 8 × 3.5 | 255 | 16,400 |
| 32 × 10 × 4 | 576 | 37,100 |
Approximate displaced volume and buoyant force at Cb = 0.45 (seawater).
Formula verified against standard naval-architecture references · Updated October 2026
Hull volume is the space inside the underwater shape of the boat, measured in cubic feet. It matters because a floating boat pushes aside its own weight in water, so the volume of hull below the waterline directly sets the maximum weight the boat can carry and still float at that draft. Get the volume right and displacement, freeboard and stability all follow correctly.
The block coefficient (Cb) is the ratio of the hull's actual underwater volume to the volume of a rectangular box with the same length, beam and draft. A full, boxy barge might have a Cb near 0.8, while a fine racing sailboat could be near 0.4. This calculator uses about 0.45 as a sensible default for typical displacement hulls so you get a reasonable estimate without full hydrostatic tables.
It is an early-design estimate, good to perhaps ten to fifteen percent for conventional hull forms. For a finished boat you would calculate areas at each station and integrate them, which is exactly what a lofting or offset-table exercise produces. For planning material and engine size the shortcut is usually close enough, but verify against real weights before you commit.
Use the length that is actually in the water at your designed load line, which is the waterline length. Overhanging bow and stern add overall length but little displaced volume, so basing the figure on length overall will overstate buoyancy. If your design has long overhangs, measure the waterline and use that.
The tool uses seawater at 64.4 lb per cubic foot, the denser and more conservative value for load planning because freshwater boats float a little higher. If your boat only ever runs on lakes and rivers, real capacity is about three percent lower, so the seawater number keeps you on the safe side when estimating maximum displacement.
Displacement is simply the weight of the boat, and by Archimedes it equals the weight of the water the hull displaces. Multiply displaced volume by water density and you get the maximum displacement for that waterline. Designers often size volume so the fully loaded boat sits at a comfortable draft with reserve freeboard, rather than filling every last cubic foot.
It depends what you are solving for. Displaced volume tells you how much the boat weighs afloat, since a floating hull displaces its own weight in water, so it is the number behind draft and load-carrying capacity. Internal volume, by contrast, governs how much room and reserve buoyancy you actually have for berths, tanks and sealed compartments. The two are close but not equal, because shell thickness, ballast and structure sit between them, and a hull can displace plenty yet feel cramped inside. Fix the displaced volume to match your target weight first, then shape the interior volume around the accommodation and flotation the boat has to provide.