What is freeboard?
Freeboard is the distance from the waterline to the freeboard deck line. The International Load Line Convention assigns a minimum summer freeboard so that the ship keeps enough reserve buoyancy and a safe working platform in a seaway. The summer load line is the deepest legal draft in salt water in a summer zone, unless a tropical allowance applies.
How this calculator works
Draft margin = Summer Load Draft − Current Mean Draft. Current freeboard = Assigned Summer Freeboard + Draft margin. If current draft is deeper than summer draft, the ship is overloaded for a summer salt-water condition. Fresh water, tropical zones and timber load lines have different marks. Always use the correct mark for the present zone and water density, and apply dock water allowance if the ship is in a river or dock.
Who should use it
Deck officers use this as a quick check before departure. It does not replace the official load line calculation, dock water allowance or the stability booklet. If the result shows overloaded, stop cargo and re-check drafts and density.