Updated November 2026 · free, works in your browser
Show the formulas
D/L = (displacement in lb ÷ 2240) ÷ (LWL in ft ÷ 100)³
SA/D = sail area in ft² ÷ (displacement in lb ÷ 64)^(2/3)
CSF = beam in ft ÷ (displacement in lb ÷ 64)^(1/3)
Hull speed = 1.34 × √(LWL in ft) knots. Metric values are converted to feet and pounds first.
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Open full version →A sailboat's design ratios turn a handful of dimensions into a quick picture of how it behaves. Enter waterline length, beam, displacement and sail area above to get the displacement/length ratio, the sail area/displacement ratio, the capsize screening value and the theoretical hull speed.
How to use the sail performance calculator
- Choose imperial (feet, pounds) or metric (metres, kilograms). Values already typed are converted for you.
- Enter length overall (LOA) and waterline length (LWL). LOA can never be shorter than LWL.
- Enter the beam at its widest point and the displacement of the boat in its sailing trim.
- Enter the sail area of the mainsail plus the working jib or genoa, as listed by the designer.
- Read the four results. They update as you type, and the link button saves your numbers in the address.
How it is calculated
All inputs are converted to feet and pounds, the units in which these ratios are traditionally defined.
- Displacement/length ratio: D/L = (displacement in lb ÷ 2240) ÷ (LWL in ft ÷ 100)³. The 2240 turns pounds into long tons.
- Sail area/displacement ratio: SA/D = sail area in ft² ÷ (displacement in lb ÷ 64)^(2/3). Dividing by 64 gives the volume of displaced seawater in cubic feet.
- Capsize screening formula: CSF = beam in ft ÷ (displacement in lb ÷ 64)^(1/3).
- Hull speed: 1.34 × √LWL in knots, the classic estimate for a displacement hull.
These are the standard formulas used in yacht design literature and in capsize screening guidance for offshore boats. They are screening tools, not a measure of seaworthiness on their own.
How to read the results
| Ratio | Range | Typical meaning |
|---|---|---|
| D/L | Below 100 | Ultra-light, fast and lively |
| D/L | 100 to 200 | Light, a sporty cruiser |
| D/L | 200 to 275 | Moderate, a balanced cruiser |
| D/L | 275 to 350 | Heavy, steady and comfortable |
| D/L | Above 350 | Very heavy, a traditional long-distance hull |
| SA/D | Below 15 | Low power, needs a breeze |
| SA/D | 15 to 20 | Moderate, typical for cruisers |
| SA/D | Above 20 | High power, performance oriented |
| CSF | 2.0 or less | Within the usual offshore screening limit |
| CSF | Above 2.0 | Wide for its weight, more exposed in heavy seas |
The bands above are widely quoted rules of thumb and sources draw the lines slightly differently, so treat them as guidance.
Frequently asked questions
What is a good sail area to displacement ratio?
For a cruising boat, values between roughly 15 and 20 are common. Lower numbers need more wind to move well, while higher ones are livelier but usually need earlier reefing.
Why does the calculator use waterline length rather than LOA?
Waterline length is the length that actually drives hull speed and wave making. Overhangs add length overall but little to the waterline when the boat is upright.
Is hull speed a hard limit?
No. It is the speed at which the boat's wave length equals its waterline length. Light, planing-capable hulls can exceed it, while heavy boats rarely reach it.
What displacement should I enter?
Use the figure for the boat loaded as you will sail it, with fuel, water, gear and crew. The brochure light-ship figure usually understates the real value.
Does a CSF above 2.0 mean the boat is unsafe?
Not by itself. It flags a boat that is wide for its weight, which matters most for long offshore passages. Coastal sailors can reasonably weigh it less heavily.
From numbers to a sailing trip
The same ratios help when you pick a boat for a holiday. A heavier, moderate-power cruiser is forgiving on a week of island hopping, while a lighter one suits sailors who want speed. Once the boat is chosen, it is worth planning where you will sleep on shore days, since a good hotel near the marina makes provisioning, weather waiting and crew changes much easier.
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