Sailboat Capsize Screen Factor Calculator
Calculate the offshore capsize screening factor for any monohull sailboat from beam and displacement.
Under 2.0 is considered offshore-capable by ORC.
The Capsize Screening Formula (CSF) is a simple measure of a monohull sailboat tendency to capsize in breaking seas.
It was developed after the disastrous 1979 Fastnet Race, in which 15 sailors died and 24 boats were lost or abandoned, largely due to repeated capsizing in breaking waves.
The formula:
CSF = beam_meters / displacement_cubic_meters^(1/3)
Where displacement is calculated from the mass: displacement_cubic_meters = displacement_kg / 1025 (seawater density)
A boat with a wide beam relative to its displacement has a high CSF, meaning it is relatively easy to knock down and slow to self-right.
A heavy, narrow-beamed offshore cruiser has a low CSF, so it is harder to capsize and quicker to recover.
Who actually wrote this formula
It came out of the joint technical study run by the United States Yacht Racing Union (USYRU, now US Sailing) and the Society of Naval Architects and Marine Engineers (SNAME) in the wake of the 1979 race. It is worth being clear about that, because the number gets loosely attributed to whichever rating body a reader has heard of. The Offshore Racing Congress does publish a stability measure, but that is STIX, the Stability Index used in ISO 12217, and it is a far more involved calculation than this one.
The threshold everyone quotes:
- CSF below 2.0: the screening cutoff, suitable for offshore ocean racing and bluewater cruising
- CSF 2.0 to 2.5: coastal passages in moderate conditions, not recommended for offshore in severe weather
- CSF above 2.5: racing or coastal use, with significant capsize risk in breaking seas
Most production coastal cruisers fall in the 2.2 to 2.4 range, which is fine for the conditions they are designed for but not appropriate for passages through potential storm zones.
Classic bluewater cruisers (Cape Dory, Hallberg-Rassy, Westsail) typically score 1.6-1.8.
Many modern wide-beam cruisers and racers score 2.4-2.8.
The CSF is a useful screening tool, not a complete stability analysis.
It does not account for ballast ratio, center of gravity height, sail area, or the full stability curve.
A low CSF is necessary but not sufficient for offshore safety. Stability curve analysis (capsize angle, range of positive stability) provides a more complete picture.
One quirk worth knowing: the formula is written in feet and pounds in most places you will meet it, as beam in feet divided by the cube root of displacement in pounds over 64. The metric form this calculator uses, beam in metres over the cube root of displacement in kilograms over 1025, gives an identical answer. The conversion factors cancel exactly, because 64 lb per cubic foot and 1025 kg per cubic metre are the same seawater density. So there is no metric and imperial version of the threshold to worry about. 2.0 is 2.0 either way.
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.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.