Capsize Screening Formula Calculator – Check Your Boat’s Capsize Screening Value

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What this tool does

The capsize screening formula is a quick, rough test that yacht designers and race organisers have used for decades to compare boats. It takes just two numbers, the boat’s maximum beam and its displacement, and turns them into a single value. Boats with a value at or under 2.0 are generally considered to pass the screening test for offshore sailing. Boats well above 2.0, especially above 2.3, are wider and lighter for their size, and the formula flags them as more capsize-prone in breaking waves. This calculator works out the value for any boat from those two inputs and explains the band it falls in.

The idea behind the formula is simple. A wide, light boat is easier for a breaking wave to roll over than a narrow, heavy boat, because the wave has more beam to push against and less weight to resist it. The formula captures that trade-off in one ratio: beam divided by the cube root of the displaced volume of seawater. Displacement divided by 64 gives that volume in cubic feet, because a cubic foot of seawater weighs about 64 pounds, and the cube root turns a volume into a length that can be compared fairly with beam.

How to use it

Enter the maximum beam in feet, measured at the widest point of the hull, and the displacement in pounds, as the maker states it for the boat in sailing condition. If your displacement is given in kilograms, multiply by 2.20462 to get pounds. If it is given in tonnes, multiply by 2,204.62. If it is given in long tons, multiply by 2,240. Press the button and read the screening value and the band it falls in, plus a plain warning that this is a screening ratio, not a stability verdict.

The formula, step by step

The formula is: capsize screening value equals beam in feet divided by the cube root of displacement in pounds divided by 64. Work through an example with a beam of 11 feet and a displacement of 12,000 pounds. Divide 12,000 by 64 to get 187.5 cubic feet of displaced seawater. The cube root of 187.5 is about 5.72. Divide the beam, 11, by 5.72, and the screening value is about 1.92. That sits under 2.0, so the boat passes the usual offshore screening test on this measure.

Try a wider, lighter boat: a beam of 13 feet and a displacement of 8,000 pounds. Divide 8,000 by 64 to get 125. The cube root of 125 is exactly 5. Divide 13 by 5 and the value is 2.60, well above the 2.3 coastal band. The formula is flagging a real design difference: the second boat is much wider for its weight, and a breaking wave has far more leverage on it. That does not mean the second boat will capsize, but it does mean it deserves a closer stability look before offshore work.

What the bands mean, and what they do not

The 2.0 line comes from offshore racing rules, which used it as a screening cut-off for ocean races. It was never meant as a law of physics, and it was never meant to be the only stability check. A boat under 2.0 can still capsize in bad conditions, and a boat over 2.0 can be perfectly safe for coastal sailing with a good crew and sensible weather limits. The bands in this tool reflect that: at or under 2.0 is the offshore screening band, 2.0 to 2.3 is a coastal band where offshore use deserves extra scrutiny, and above 2.3 fails the usual offshore screening test.

A worked example with your own boat

Suppose your boat has a beam of 10.5 feet and a displacement of 15,000 pounds. Divide 15,000 by 64 to get 234.4 cubic feet. The cube root is about 6.17. Divide 10.5 by 6.17 and the value is about 1.70, comfortably under 2.0. Now add cruising gear until displacement reaches 17,000 pounds. The cube root rises to about 6.43, and the value falls to about 1.63. Loading the boat helped the screening value, though it also made the boat slower and lower in the water, which shows why no single number tells the whole story of a boat.

Limits and safety

This calculator does not replace seamanship, the boat maker’s guidance, or a professional stability assessment. It must never be used to declare a boat safe for offshore sailing, because it ignores the very factors that often decide whether a boat recovers from a knockdown. It also uses a single displacement figure, while real displacement changes with loading, and a single beam figure, while real hull shape matters enormously. Treat the result as a conversation starter with a designer or surveyor, not as a verdict.

If your boat screens above 2.0 and you sail offshore, that is a prompt to learn more, not a reason to panic and not a reason to ignore it either. Check the fuller stability data, sail within sensible weather limits, keep the boat watertight, practise heavy-weather techniques, and consider what a capsize would mean for your boat: can it recover, can the crew stay with it, and can you call for help? Those questions matter far more than any single ratio.

Common questions

Can a boat over 2.0 still sail offshore? Yes, many do, and many race rules now use fuller stability indices instead of this screening formula alone. The value is a flag for closer inspection, not a ban. Does a lower value always mean a safer boat? Not always. A very narrow, very heavy boat can screen beautifully and still be a poor sea boat in other ways, such as rolling violently or being hard to handle. Safety is a whole-boat property, and no single ratio captures it.

Versión en español: Calculadora de Fórmula de Estabilidad Antivuelco