Estimate the safe freeboard for your boat based on its dimensions.
Coastal cruising sailboat
Offshore fishing boat
Inshore flat-water skiff
feet
feet
feet
"My Boat Plans" is a collection of over 500 boat plans, from simple dinghies to larger cruisers. It includes step-by-step guides, video tutorials, and more.
Download NowFreeboard is the height of the deck above the waterline, and it is the boat's first defence against coming aboard in a seaway. Plenty of freeboard keeps water out, gives reserve buoyancy and lets a boat take waves without shipping them, while too little leaves the deck awash at the worst possible moment.
It falls straight out of hull depth and draft: the deeper the hull sits above its waterline and the less of it buried, the more freeboard remains. A rough safety floor scales with boat length, and the sensible target sits comfortably above that minimum so the boat has margin when loaded and in waves.
Enter boat length, hull depth and draft, and the calculator finds the current freeboard, an estimated minimum and recommended value for the length, and the safety margin, so you can judge whether the boat is dangerously low, marginal or adequately freeboarded.
Current Freeboard = Hull Depth − Draft; Minimum ≈ Length × 0.05 + 0.25; Recommended = Minimum × 1.25| Freeboard vs Minimum | Status | Implication |
|---|---|---|
| Below minimum | Dangerously Low | Overloaded for length, unsafe |
| Minimum to recommended | Marginal | OK calm, short in heavy seas |
| Above recommended | Good | Adequate reserve and dryness |
| Very high | Tender/stiff trade | Dry but more windage aloft |
Freeboard status and its meaning.
Formula verified against standard naval-architecture references · Updated October 2026
A useful safety floor is roughly five percent of the boat's length plus a small constant, with a recommended value about a quarter above that so there is margin. A 30 foot boat wants around 1.75 feet minimum and nearer 2.2 feet to be comfortable. Enter length, depth and draft and the calculator compares your actual freeboard to those thresholds and grades it dangerously low, marginal or good.
For dryness and reserve buoyancy, largely yes, but not without cost. Extra freeboard raises the deck, adds weight and top-side windage, catches more cross-wind, and can make the boat tender or hard to handle in a blow if pushed aloft. High-sided offshore boats are dry and safe because they pair freeboard with stability and strength, not because height alone is a virtue.
Draft is how deep the hull sits below the waterline; freeboard is how high the deck stands above it. They come from the same hull depth: freeboard equals depth minus draft, so a boat with fixed depth trades one against the other as load changes. More load deepens the draft and robs freeboard, which is exactly why an overloaded low boat suddenly ships water.
Because there is less deck between the waterline and the sea surface, so waves and bow spray reach aboard far sooner, and the boat has less reserve buoyancy before it goes under green water. Low freeboard suits calm, sheltered water and traditional open boats that spill water through scuppers, but it confines a vessel to lighter conditions than a higher-sided design of the same length.
Within reason, by raising the sheer line, adding a toerail or bulwark, fitting a higher coaming or a raised foredeck, all of which keep deck water off and add reserve. Do it with the weight aloft and the effect on stability in mind, and keep drainage so shipped water escapes. Major freeboard change alters the whole boat, so weigh the dryness gained against the stiffness and windage cost.
Vertically from the loaded waterline up to the main deck edge, ideally at the point where a boat would ship water, commonly amidships and forward. Use the loaded waterline, not the light one, because that is the condition where freeboard is lowest and matters most. The calculator derives it from hull depth minus loaded draft, so feed in the real, laden figures.
This tool uses a simplified approximation of standards such as ABYC and load-line practice, where required freeboard scales with length. Formal rules add corrections for sheer, flare, bow height and intended service, and classification bodies set precise values for larger vessels. Use the estimate to sense-check small craft and consult the actual standard for anything where compliance genuinely matters.