Antifouling Bottom Paint Calculator (Wetted Surface Area, Litres or Gallons & Cost)
Estimate a boat's underwater (wetted) surface area from waterline length, beam and draft for its hull type, then the litres or gallons of antifouling for the number of coats at the paint's coverage, the cans to buy, cost, and the extra for a boot stripe and touch-ups.
How it works
wetted surface ≈ LWL × (beam + draft) × factor (fin keel 0.75, full keel 0.85, planing power 0.90, displacement 0.95, catamaran 0.65 per hull) · litres = area ÷ coverage × coats + waterline band + 10% waste · 1 US gal = 3.785 L; 1 m²/L ≈ 40.7 ft²/gal
The paint you need depends on the area under the water, which the classic yard formula estimates from waterline length, beam and draft with a factor for hull shape — good to within about 10% for most boats, which is what you need to buy the right number of cans. Antifouling should go on at the film thickness the maker specifies (thin coats do not release biocide for the full season), so plan real coats at the label coverage, an extra band at the waterline where growth is worst, and a margin for what stays in the tray.
Worked example
A fin-keel sailboat with 9.5 m waterline, 3.2 m beam and 1.7 m draft has roughly 34.9 m² of wetted surface. Two coats at 9 m²/L are about 7.8 L, plus a waterline band and 10% waste — about 8.9 L in total — so four 2.5 L cans (10 L) at $45/L come to $450, with a litre spare for touch-ups; expect roughly six hours of rolling.
Frequently asked questions
How many coats of antifouling?
Two full coats for most paints, with a third at the waterline, bow, keel leading edge and rudder where water flow scrubs the paint fastest. On a well-covered previous season one refresher coat can do for hard paints; ablatives wear off and need the full two.
Can I paint over old antifouling?
Yes if it is sound and compatible (ablative over hard is fine after sanding; hard over ablative is not). Flaking, chalky or unknown paint should be sanded back or stripped, and bare gelcoat primed with an epoxy barrier coat first.
Why do I use more paint than the can says?
Label coverage is for a smooth surface and a full wet film; rough old paint, a porous surface, a heavy roller and thinning all use more. Budget 10–20% extra and keep the leftover for the waterline mid-season.
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Boating figures are planning estimates based on published rules of thumb and standards (USCG, ABYC, manufacturer tables). Your boat's capacity plate, builder documentation and local regulations govern; conditions at sea vary.