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Displacement / Length Ratio Calculator

Calculate a boat's displacement to length ratio (D/L = disp in long tons ÷ (0.01 × LWL)³), classify it from ultralight to heavy displacement, compare with hull speed and the speed/length ratio it can realistically reach, and see the displacement that would put it in a different class.

Displacement

Waterline length (LWL)

Length overall (optional)

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How it works

D/L = (displacement ÷ 2,240) ÷ (0.01 × LWL)³ with displacement in lb (long tons) and LWL in feet; hull speed = 1.34 × √LWL

Displacement/length compares a boat's weight with the cube of its waterline length, removing size so that a dinghy and a schooner can be judged on the same scale of heaviness. Light boats (under 180) are slippery, quick to accelerate and can surf past hull speed, but they pitch more and their performance suffers as they are loaded; heavy boats (over 270) push a bigger wave, are wedded to hull speed, and in exchange carry stores easily and move gently in a seaway. The ratio also explains why designers stretch waterlines: because length is cubed, adding two feet of LWL to a 30-footer at the same weight drops D/L by nearly 20%.

Worked example

An 18,000 lb boat on a 30 ft waterline has D/L = 8.04 ÷ 0.027 = 298 — heavy displacement, in the traditional-cruiser band. Its hull speed is 7.3 kt and it will realistically cruise at 1.25 × √30 ≈ 6.8 kt; the same hull would need to weigh under 16,300 lb to be classed as moderate.

Frequently asked questions

Does the ratio change when the boat is loaded?

Yes — add the cruising payload to displacement. Waterline also lengthens slightly as the boat sinks, which offsets a little of the increase on boats with overhangs.

Why is D/L used with SA/D?

D/L says how heavy the boat is for its length; SA/D says how much sail drives that weight. A heavy boat with a big rig (high D/L, high SA/D) and a light boat with a small rig can both be well balanced.

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Sail Area / Displacement Ratio Calculator

Calculate a sailboat's sail area to displacement ratio (SA/D = SA ÷ (disp/64)^⅔), interpret it from under-canvassed cruiser to racer, see the sail area needed for a target ratio, the upwind versus downwind sail plan, and how loading the boat for cruising lowers the number.

Boat Hull Speed Calculator

Estimate a displacement boat’s hull speed from its waterline length — the natural speed limit set by its own bow wave.

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Calculate Ted Brewer's motion comfort ratio from displacement, waterline and overall length and beam, interpret it from light racer to heavy offshore cruiser, and see how adding payload or beam changes the predicted motion in a seaway.

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Calculate the US Sailing capsize screening formula (beam ÷ (displacement/64)^⅓) used to screen offshore-race entries, with the 2.0 threshold, the maximum beam or minimum displacement that would pass, the ballast ratio, and notes on limits of the formula for modern wide, light hulls.

Design ratios are comparative indicators from published yacht-design practice (Brewer, Gerr, US Sailing), not seaworthiness certifications. Use the designer's data, stability curves (STIX/AVS) and the boat's actual loaded displacement; sea state, crew and preparation matter more than any ratio.