Boat Displacement Calculator

A boat’s displacement is its weight: afloat, a boat displaces its own weight of water. The American Practical Navigator (Bowditch) calls displacement the amount of water displaced by a vessel afloat, and so a measure of its actual weight. A boat’s light displacement is its weight empty; with fuel, water, people and stores aboard it is the loaded displacement, and the difference between the two is the deadweight. The calculator above adds your load to the light displacement, gives the volume of water the boat displaces in seawater or fresh water, and estimates how much deeper the load puts it.

Light vs Loaded Displacement

Rob Mazza, writing in Good Old Boat, calls the published displacement the most problematic number in yacht design. It is often the designed displacement rather than the real one, worked out for an empty boat or with half-full tanks and no provisions, and boats get heavier as they age and gather gear, so a fully laden cruising boat is usually much heavier than its published figure. Start from the published or weighed light displacement and add what is aboard. For the example sailboat of 15,000 lb:

Load	Weight used	Pounds
Diesel, 30 gal	7.01 lb per gal	210
Fresh water, 40 gal	8.34 lb per gal	333
Four people	185 lb each	740
Gear and stores		500
Added load (deadweight)		1,784

The boat displaces 16,784 lb loaded, 11.9% more than light, or 7.49 long tons of 2,240 lb. Fuel and water weights come from the densities in the Fuel and Water Weight Calculator: 6.18 lb per gallon of gasoline, 7.01 of diesel and 8.34 of fresh water. The 185 lb per person is the U.S. Coast Guard’s assumed average weight per person for the stability of passenger vessels, a planning figure; weigh your crew and gear if you can. For a trailer boat, the Boat Weight Calculator works the same way from the dry weight and adds the trailer.

How Much Water a Boat Displaces

By Archimedes’ principle, a floating boat is pushed up with a force equal to the weight of the water it displaces, so it sinks until it has displaced its own weight (U.S. Naval Academy, Principles of Ship Performance). Bowditch gives a cubic foot of seawater as 64 lb and of fresh water as 62.428 lb, so the loaded example displaces 16,784 ÷ 64 = 262.2 cubic feet (7.43 cubic meters) of seawater and 268.9 cubic feet of fresh water. The same boat displaces 2.5% more volume in fresh water, so it floats a little deeper on a lake or river than at sea. Bowditch’s displacement ton, a long ton, is the weight of 35 cubic feet of seawater.

How Much Deeper a Loaded Boat Floats

Naval architects measure how much weight sinks a hull by one inch. The Naval Academy’s course gives the tons per inch immersion as the waterplane area in square feet ÷ 420, from 64 lb per cubic foot of seawater, and the parallel sinkage as the weight added divided by it; that assumes the weight goes on at the center of flotation, so the boat does not trim, and that the waterplane area hardly changes over the extra depth. In pounds, it is the waterplane area × 64 ÷ 12. The calculator estimates the waterplane area with Ted Brewer’s rule of thumb: for a normal monohull sailing yacht it is close to 0.65 × waterline length × waterline beam, raised to 0.70 for powerboats and other boats with a submerged transom. The waterline beam is the hull’s width at the waterline, which designers rarely publish (Brewer); with the maximum beam in its place, the pounds per inch come out too high and the sinkage too low. For the example, 0.65 × 29 × 9.5 = 179.1 square feet gives 955 lb per inch, so the 1,784 lb load puts the boat about 1.9 inches deeper. It is an estimate for the boat at rest: a planing boat at speed is held up by the water’s dynamic pressure on the hull, and acts like a displacement boat only at slow speed (Naval Academy).

Displacement is also the weight in a sailboat’s performance ratios: the Sailboat Ratios Calculator works out the displacement/length and sail area/displacement ratios, which come out differently with the light and the loaded figure. The weight a boat may carry is on its capacity plate; the Boat Capacity Calculator explains how the Coast Guard works it out for small boats.

Frequently Asked Questions

Sources

• NGA, The American Practical Navigator (Bowditch), Vol. II (2019): glossary, tonnage (displacement as the amount of water displaced and a measure of actual weight; deadweight as the difference between loaded and light displacement; warships by displacement tonnage); Calculations and Conversions (1 cubic foot of seawater = 64 pounds, of fresh water 62.428 pounds; 1 displacement ton = 35 cubic feet of seawater = 1 long ton) (read Oct 22, 2026).
• U.S. Naval Academy, EN400 Principles of Ship Performance, Chapter 2: Hull Form and Geometry: Archimedes’ principle, displacement ships, planing vessels, displacement (2.10.1), tons per inch immersion and parallel sinkage (2.10.4) (read Oct 22, 2026).
• Ted Brewer, The Comfort Ratio, Good Old Boat, Issue 32 (September/October 2003): the waterline area of a normal monohull sailing yacht close to 0.65 × waterline length × waterline beam, 0.70 for powerboats and craft with a submerged transom; designers rarely provide the waterline beam (read Oct 22, 2026).
• Rob Mazza, Ratio rationale, Good Old Boat, Issue 93 (November/December 2013): the published displacement, often the designed displacement for an empty boat or half-full tanks, lighter than the real boat (read Oct 22, 2026).
• eCFR, 46 CFR 170.090, Calculations: assumed average weight per person, 185 pounds (read Oct 22, 2026).