Estimate underwater (wetted) hull area for bottom paint planning from length, beam, draft, and a hull-shape factor you choose. Optional gallons from a coverage rate you enter.
Use for a first-pass antifouling order before you have a measured wetted surface.
Compare to the worked example; fin-keel boats often use a lower factor than full-bodied hulls.
Not a brand coverage calculator — enter your can’s sq ft/gal if you want gallons.
area ≈ length × (beam + draft) × factor. Optional gallons ≈ (area × coats) ÷ coverage_sqft_per_gal.
area ≈ L × (B + D) × factor
gal ≈ (area × coats) ÷ coverage (optional)
Geometry + your factor only — not Coppercoat/TotalBoat OEM wizards.
Defaults: 34 × (11+4) × 0.75 = 382.5 sq ft. Two coats at 400 sq ft/gal → ≈ 1.91 gal.
Factor 1.0 → 510 sq ft.
From the defaults (Length (ft) — LOA or LWL = 34; Beam (ft) = 11; Draft (ft) = 4; Hull factor = 0.75; Coverage (sq ft/gal, optional) = 400; Coats = 2), the calculator shows:
382.5 sq ft
factor 0.75 · ≈ 1.91 gal for 2 coat(s)
area ≈ 34 × (11+4) × 0.75 = 382.50 ft²
gal ≈ (382.50 × 2) ÷ 400 = 1.913 gal
Now raise Length (ft) — LOA or LWL from 34 to 51 and keep the other entries. The result becomes:
573.8 sq ft
factor 0.75 · ≈ 2.87 gal for 2 coat(s)
area ≈ 51 × (11+4) × 0.75 = 573.75 ft²
gal ≈ (573.75 × 2) ÷ 400 = 2.869 gal
Comparing the two runs tells you how sensitive the answer is to that one measurement.
Assumptions: simple wetted-area rule of thumb; length is waterline when you can measure it.
Limits: multihulls, steps, and long keels differ — raise factor or measure panels.
Responsibility: follow paint TDS for coverage, coats, and legal use in your waters.
Waterline length tracks underwater area better. If you only have LOA, expect a modest overestimate.
Full-bodied powerboats often near 1.0; medium fin/skeg ~0.75; light fin ~0.65. Enter what matches your hull — not invented here as a secret default table beyond the hint.
Above-water canvas is a different tool — boat canvas area.