Capsize Screening Formula Calculator
Calculate the conventional capsize screening value for a displacement monohull sailboat from its maximum beam and displacement. Use the result as a rough comparison—not as a safety rating or voyage decision.
How the two inputs move this result
This arithmetic illustration changes one input by 10% while holding the other fixed. It is not a design recommendation or safety prediction.
The 2.0 line is a conventional historical screening benchmark, not a boundary between safe and unsafe boats.
How to use the calculator
The result is only as meaningful as the measurements supplied. Compare boats using consistent definitions and loading conditions.
Capsize screening formula and units
The conventional formula uses maximum beam in feet and displacement in pounds:
The historical convention treats 64 pounds as the approximate weight of one cubic foot of seawater. Dividing weight by 64 produces an estimated displaced volume in cubic feet.
The cube root converts the estimated volume into a length-like value. Dividing beam by that value produces the dimensionless screening ratio.
A narrower or heavier boat produces a lower value in this formula. A wider or lighter boat produces a higher value. This arithmetic relationship does not describe every source of stability.
Worked example
Consider a displacement monohull with a maximum beam of 12 feet and a displacement of 8,000 pounds.
The value is above the conventional 2.0 screening benchmark. That observation alone does not calculate capsize probability or establish that the boat is unsafe.
How to interpret the benchmark
The conventional 2.0 benchmark is best treated as a prompt for further investigation, not as a certification line.
| Calculated value | Careful interpretation | What to do next |
|---|---|---|
| 2.00 or below | The boat is at or below the conventional screening benchmark based only on maximum beam and displacement. | Review complete stability data, vessel condition, loading, design, and intended operating conditions. |
| Above 2.00 | The boat is above the conventional screening benchmark based only on maximum beam and displacement. | Investigate why the design produces the value and use more complete stability information before drawing conclusions. |
What the number leaves out
Two vessels can receive the same CSF value while behaving very differently because the formula cannot see their geometry, structure, loading, or recovery characteristics.
Freeboard, hull shape, deck openings, cockpit volume, appendages, and reserve buoyancy are absent.
Total displacement is included, but ballast location, vertical center of gravity, tank levels, and carried equipment are not.
The result does not provide angle of vanishing stability, a GZ curve, righting energy, inversion recovery, or downflooding angles.
Wave shape, wind, damage, watertight integrity, maintenance, loading changes, and seamanship are not represented.
The historical screen is associated with displacement monohull sailboats. Do not transfer the 2.0 benchmark directly to multihulls, dinghies, or powerboats.
Do not use this result alone to approve offshore use, select a voyage, purchase a boat, or replace professional stability information.
Frequently asked questions
Is a CSF result below 2.0 proof that a boat is safe offshore?
No. It only means the beam-and-displacement result is at or below the conventional screening benchmark. Safety and suitability require much broader information.
Should I use maximum beam or waterline beam?
Use the vessel’s maximum beam for this formula. The input should come from a reliable specification and use a consistent definition when comparing boats.
Which displacement should I enter?
Use the displacement for a known loading condition and record that condition. Light, design, and fully loaded displacement can produce different results.
Can I enter metres and kilograms?
Yes. Select metric units. The calculator converts metres to feet and kilograms to pounds before applying the conventional formula.
Why does this version not calculate ballast ratio or comfort ratio?
Those are separate measures with different purposes and input requirements. Keeping this page focused prevents unrelated results from being mistaken for part of the capsize screening formula.
Does bokog store the measurements I enter?
The calculation is designed to run locally in the browser. The entered measurements are not submitted to a bokog calculation API.
Use the result as the beginning of a stability review
Confirm the boat’s measurements, identify its loading condition, review complete manufacturer or professionally prepared stability information, and consider the actual vessel, route, weather, maintenance, and crew. A simplified ratio cannot make that decision for you.