Boat Freeboard Calculator

Boat Freeboard Calculator

Freeboard is the hull height remaining above the waterline - a critical safety margin against waves, wake, and spray coming over the side of the boat.

Freeboard = Hull Depth - Draft

Example

36-inch hull depth, 12-inch draft:

Freeboard = 36 - 12 = 24 inches (66.7% of hull depth - generous freeboard)

Why Freeboard Shrinks as You Load the Boat

Freeboard decreases as a boat is loaded with more weight - passengers, gear, fuel - since additional weight pushes the hull deeper into the water, increasing draft and correspondingly reducing freeboard. This is exactly why overloading a boat beyond its rated capacity directly reduces this safety margin, sometimes dangerously so in rough water conditions where waves can more easily come over a reduced freeboard.

Reading Freeboard as a Percentage of Hull Depth

Expressing freeboard as a percentage of total hull depth (rather than just an absolute measurement) helps compare boats of different sizes on equal footing - a small boat and a large boat with the same absolute freeboard in inches can have very different relative safety margins depending on their overall hull depth.