Boat Displacement Calculator
Calculate boat displacement in pounds and tons, waterline length, and D/L ratio.
Classifies your vessel as ultralight, light, moderate, or heavy displacement.
Displacement is the weight of water a boat pushes aside when floating. By Archimedes’ principle, this equals the total weight of the boat and everything aboard. It is the single most important number in boat design, because speed potential, stability and seakeeping all follow from it.
Archimedes’ Principle
Displacement (lbs) = Volume of submerged hull (cubic feet) × Water density
Seawater density: 64.0 lbs/ft³ (1025 kg/m³) Freshwater density: 62.4 lbs/ft³ (1000 kg/m³)
Displacement-to-Length Ratio (D/L)
The D/L ratio is the standard comparison metric for how heavy or light a boat is relative to her length:
D/L = Displacement (long tons) / (0.01 × LWL in feet)³
Where: 1 long ton = 2,240 lbs.
| D/L Range | Classification | Examples |
|---|---|---|
| < 100 | Ultra-light | Racing multihulls, skiffs |
| 100–150 | Light | Racing sailboats, sport boats |
| 150–250 | Moderate | Coastal cruisers, family sailboats |
| 250–350 | Heavy | Traditional cruisers, motorsailers |
| > 350 | Very heavy | Full-keel bluewater yachts, workboats |
Hull Speed Formula
The theoretical maximum speed for a displacement hull is:
Hull speed (knots) = 1.34 × √(LWL in feet)
A boat with 30 ft LWL: hull speed = 1.34 × √30 = 7.34 knots. A boat with 40 ft LWL: hull speed = 1.34 × √40 = 8.47 knots.
Lighter boats (lower D/L) can exceed hull speed more easily and may plane. Heavier boats are effectively limited to hull speed.
Sail Area to Displacement Ratio (SA/D)
SA/D = Sail area (sq ft) / (Displacement in lbs / 64)^(2/3)
| SA/D Range | Performance |
|---|---|
| < 14 | Underpowered, needs engine in light air |
| 14–17 | Average, good balance of comfort and performance |
| 17–20 | Fast, responsive in moderate conditions |
| > 20 | Racing, overpowered for casual sailing |
Worked Example: 35-foot Coastal Cruiser
LWL: 30 ft, Displacement: 14,000 lbs. D/L = (14000 / 2240) / (0.01 × 30)³ = 6.25 / 0.027 = 231.5, which lands in the moderate band. Hull speed = 1.34 × √30 = 7.34 knots. With 550 sq ft sail area: SA/D = 550 / (14000/64)^(2/3) = 550 / 36.3 = 15.1, an average balance of comfort and performance.
Note that SA/D always divides by 64, the seawater figure, whichever water you sail in. That is how the ratio is defined, so two boats stay comparable regardless of where they float.
Prismatic Coefficient
The prismatic coefficient (Cp) describes hull shape, specifically how much of the underwater volume sits out at the ends rather than in the middle:
Cp = Displaced volume / (Maximum cross-section area × LWL)
Optimal Cp for hull speed: 0.55 to 0.60. Higher Cp (0.60 to 0.65): better for higher speed-to-length ratios.
The calculator below does not compute Cp, because it needs the maximum immersed cross-section area and that is not something most owners have to hand. It is here as a reference for reading a designer’s spec sheet.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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