Displacement to Length Ratio Calculator
The displacement to length ratio, or DLR, is a classic naval architecture number that describes how heavy a boat is relative to its waterline length. It helps classify hulls along a spectrum from light, easily driven racing designs to heavy, sea-kindly cruisers. This calculator uses the standard formula: divide the displacement, expressed in long tons of 2,240 pounds, by the cube of one one-hundredth of the waterline length in feet. The cube term means length has a powerful effect, so a longer waterline at the same weight sharply lowers the ratio. Enter the displacement in long tons and the waterline length in feet, and the tool returns the dimensionless DLR. Lower values, roughly under 200, indicate light, performance-oriented hulls; values in the mid hundreds indicate moderate cruisers; and high values, above 300 or 400, indicate heavy, traditional designs. The ratio is a comparison aid, not a verdict on quality, since the right balance always depends on whether a boat is built for speed, for offshore comfort, or for carrying a heavy cruising load. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator defaults so you can follow each step.
The DLR is displacement in long tons / (0.01 x waterline length)^3. A 10 long-ton boat with a 25 foot waterline has a DLR of 640.00, a heavy-displacement figure.
Displacement to length ratio formula
DLR = D / (0.01 x LWL)^3
D = displacement in long tons (2,240 lb each)
LWL = waterline length in feet
(the result is dimensionless)
Because the length term is cubed, a longer waterline lowers the ratio steeply. Convert pounds to long tons by dividing by 2,240 before entering displacement.
Worked example
Suppose the displacement is 10 long tons and the waterline length is 25 feet.
- Length term: 0.01 x 25 = 0.25
- Cube it: 0.25^3 = 0.015625
- DLR: 10 / 0.015625 = 640.00
The displacement to length ratio is 640.00, which falls in the heavy-displacement range. These are the calculator's default inputs, so the result matches the widget exactly.
Displacement to Length Ratio Calculator: frequently asked questions
What does the DLR tell you?
It indicates how heavy a boat is for its waterline length, a proxy for whether the hull is light and easily driven or heavy and sea-kindly. Low ratios suggest performance designs that accelerate and plane more readily; high ratios suggest traditional cruisers that carry stores well and ride comfortably in a seaway.
What is a typical DLR range?
As a rough guide, values under about 200 are light, 200 to 300 are moderate, and above 300 to 400 are heavy. Ultralight racing hulls can be well below 100. These bands are general comparisons, not strict categories, and naval architects interpret them alongside other ratios.
Why is displacement in long tons?
The traditional formula uses long tons of 2,240 pounds, the unit historically used in naval architecture. If you know displacement in pounds, divide by 2,240 to get long tons before entering it. Using the wrong unit gives a ratio that cannot be compared with published figures.
Why does waterline length matter so much?
The length term is cubed in the denominator, so it has an outsized effect. A small increase in waterline length sharply reduces the ratio at the same weight, which is one reason longer boats of similar build tend to be faster and more easily driven.
Is a higher or lower DLR better?
Neither is universally better; it depends on purpose. A low ratio favors speed and responsiveness, while a high ratio favors carrying capacity and comfortable motion offshore. The right value is the one suited to how the boat will be used. NOAA provides extensive marine and boating safety resources.
Official sources
- Marine, boating and navigation resources: US National Oceanic and Atmospheric Administration (NOAA). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.