Capsize Screening Formula Calculator for Sailboats
Boat measurements
Screening result
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Formula and units
The conventional capsize screening formula is:
CSF = Beam_ft ÷ (Displacement_lb ÷ 64)^(1/3)
Dividing weight by 64 lb/ft³ approximates displaced seawater volume. Its cube root produces a length, so maximum beam divided by that value is dimensionless.
Metric entries: Metres and kilograms are converted to feet and pounds before the legacy formula is applied. Switching units does not materially change the result.
How to use the calculator
- Select imperial or metric units.
- Enter maximum beam from a reliable specification.
- Enter displacement for a stated loading condition.
- Calculate and review the substituted values.
- Use CSF only as one coarse historical comparison.
How to interpret the result
The Cruising Club of America-derived screen is commonly described with 2.0 or lower as the conventional benchmark. Lower values result from less beam, more displacement, or both. This relationship is arithmetic context—not proof of safer design.
Two boats with the same beam and displacement receive the same CSF even if one has deep ballast and the other carries weight high in the hull. CSF also omits hull form, reserve buoyancy, downflooding openings, keel and ballast geometry, stability curves, structural condition, sea state, and crew actions.
Do not use “at or below 2.0” as a voyage decision. Modern offshore requirements use fuller evidence such as STIX, angle of vanishing stability (AVS), righting energy, and rating-system stability indices. Consult the boat's approved stability information, current event rules, and a qualified naval architect or surveyor where safety decisions depend on stability.
Frequently asked questions
Why is 64 used in the formula?
It is the conventional approximate weight of one cubic foot of seawater in pounds. Displacement in pounds divided by 64 estimates displaced volume in cubic feet.
What does a result of 2.0 or less mean?
It is at or below the formula's conventional historical benchmark. It is not a pass certificate and does not prove offshore suitability, self-righting ability, or freedom from capsize.
Which beam measurement should I use?
Use maximum overall hull beam from a reliable source. Do not substitute beam at the waterline or a width measured at an arbitrary station.
Should I use design or loaded displacement?
Use a displacement with a clearly stated definition and compare boats on the same basis. A representative loaded condition may differ substantially from brochure or lightship displacement. Added stores can also change centre of gravity, which CSF ignores.
Would adding weight improve stability because it lowers CSF?
Not necessarily, and adding weight based on this result is unsafe. Weight location matters: added mass high in the boat can reduce stability even though the formula's number falls. Loading can also affect freeboard, structure, trim, and downflooding.
Can I use CSF for multihulls or dinghies?
The traditional screen was developed around conventional displacement monohulls. It does not characterize multihull or dinghy capsize and recovery behavior, and its 2.0 benchmark should not be transferred to them.
How is CSF different from AVS or STIX?
CSF uses only maximum beam and displacement. AVS comes from the boat's righting-arm behavior, while STIX combines several stability and recovery factors. They are not interchangeable.
Limits and safety disclaimer
- The formula is a screening ratio, not a capsize prediction or probability model.
- It does not include centre of gravity, ballast ratio or depth, hull shape, deckhouse volume, downflooding, free-surface effects, or righting-arm data.
- A lower number does not authorize offshore use, establish compliance, or replace vessel-specific stability documentation.
- Measurement source and loading condition can materially change the result.
Safety disclaimer: This calculator is for education and consistent preliminary comparison only. Do not use it alone to choose a vessel, plan a passage, set loading, satisfy race or regulatory requirements, or make operational decisions.
Methodology and sources
Last reviewed: August 2, 2026. Formula, maximum-beam definition, 64 lb/ft³ convention, historical benchmark, and modern-stability context were checked against: