Capsize Screening Formula Calculator
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.
How it works
CSF = beam (ft) ÷ (displacement (lb) ÷ 64)^⅓; pass ≤ 2.0; ballast ratio = ballast ÷ displacement
After the 1979 Fastnet disaster the Cruising Club of America and US Sailing looked for a quick way to flag boats whose hull form made them prone to capsize and slow to re-right, and settled on beam relative to the cube root of displaced volume: wide, light boats have high initial (form) stability but also stay inverted once rolled, whereas narrow, heavy boats roll more easily but recover. A value of 2.0 or less became the screening criterion for offshore racing entries. The formula is deliberately crude — it knows nothing about how low the ballast is or how buoyant the coachroof — so modern designs are assessed with the ISO stability index and angle of vanishing stability, but the CSF remains a useful first look at any boat contemplated for ocean passages.
Worked example
A 12 ft-beam, 18,000 lb boat scores 12 ÷ (281.25)^⅓ = 12 ÷ 6.55 = 1.83 — passing the 2.0 screen comfortably; it could be up to 13.1 ft wide at that weight. With 7,000 lb of ballast its ballast ratio is 39%, typical for a cruiser.
Frequently asked questions
Is a CSF over 2 dangerous?
It means the boat relies on form stability. Plenty of coastal cruisers and racer-cruisers score 2.0–2.3 and are perfectly good boats for their purpose; for ocean races the organisers usually want the ISO STIX instead, where such boats may still qualify.
What about multihulls?
The formula does not apply — catamarans have enormous beam and form stability by design and no righting moment inverted; they are assessed by different criteria (bruce number, righting moment vs sail carrying power).
Related calculators
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.