What Is a Freeboard Percentage Calculator?
Freeboard is the height of a hull's side that sits above the waterline. This calculator divides your measured freeboard by the boat's overall hull depth (freeboard plus submerged depth) to express freeboard as a percentage, which makes it easy to compare hulls of different sizes on an apples-to-apples basis.
There's no single regulatory freeboard percentage requirement for recreational boats, but naval architects and surveyors commonly use this ratio as a quick, comparable indicator of reserve buoyancy and expected dryness in a seaway.
How to Read Your Results
Freeboard Percentage
Freeboard divided by total hull depth, times 100. A higher percentage generally means more hull sits above the water, offering more margin before water comes over the side.
Submerged Depth
The portion of the hull depth below the waterline — total hull depth minus freeboard.
General Comparison
A rough, non-regulatory descriptive band showing where your ratio falls relative to typical recreational hull types, from low-sided runabouts to high-sided offshore boats.
Real-World Example
A boat has 24 inches of freeboard measured from the waterline to the gunwale, on a hull with an overall depth of 48 inches from gunwale to keel.
| Value | Amount |
|---|---|
| Freeboard height | 24 in |
| Overall hull depth | 48 in |
| Submerged depth | 24 in |
| Freeboard percentage | 50% |
With exactly half the hull depth above the waterline, this boat falls on the higher end of typical recreational freeboard ratios, suggesting a relatively dry ride in moderate chop.
Tips for Using This Calculator
- Measure at a realistic load — freeboard drops as you add fuel, gear, and passengers, so an empty-boat measurement will overstate your usual freeboard.
- Compare bow and stern separately if your boat trims noticeably, since freeboard often differs along the length of the hull.
- Use consistent reference points (same rail line, same keel point) each time you measure so comparisons over time stay meaningful.
- Remember this is a general guideline, not a stability or safety certification.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team