Boat Displacement Calculator
Archimedes' principle says a floating boat displaces a weight of water equal to its own weight. From the submerged hull volume and the water density you can find that displacement directly. Enter the immersed volume in cubic metres and choose or edit the water density (sea water near 1,025 kg per cubic metre, fresh water near 1,000). The calculator returns the displacement in kilograms, metric tonnes, and long tons.
Displacement formula
Displacement (kg) = submerged volume (m^3) * water density (kg/m^3)
Metric tonnes = kg / 1,000
Long tons = kg / 1,016.0469088
One long ton is 1,016.0469088 kilograms, the unit traditionally used for ship displacement.
Worked example
- Submerged volume 10 cubic metres in sea water at 1,025 kg/m^3.
- Displacement = 10 * 1,025 = 10,250.00 kg.
- That is 10.25 metric tonnes, or about 10.09 long tons.
Boat displacement: frequently asked questions
What is boat displacement?
Displacement is the weight of the water a floating boat pushes aside, which by Archimedes' principle equals the boat's own weight. It is found from the submerged hull volume times the water density: displacement = volume * density. For a floating vessel, displacement and total weight are the same number.
What density should I use for sea water?
Sea water density is commonly taken as about 1,025 kilograms per cubic metre (1.025 tonnes per cubic metre), and fresh water as 1,000 kilograms per cubic metre at typical temperatures. Density varies with salinity and temperature, so this calculator lets you edit it.
Why does a boat float deeper in fresh water?
Fresh water is less dense than sea water, so a given submerged volume displaces less weight. To displace its full weight the hull must sink slightly deeper in fresh water, immersing more volume. This is why a vessel shows a higher waterline at sea than in a river.
Is displacement the same as the boat's weight?
For a freely floating boat in equilibrium, yes: the weight of displaced water equals the boat's weight. This calculator finds the displacement from the submerged volume, which equals the loaded weight the hull can support at that immersion.
Official sources
- NOAA: why is the ocean salty and dense (sea water density).
- NIST: unit conversions (kilogram, tonne, long ton).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.