Boat Bottom Paint Calculator
Estimate gallons of antifouling paint needed for your boat hull.
Calculated from waterline length, beam, and draft with coverage rate and number of coats.
Calculating Wetted Surface Area
The amount of antifouling paint you need depends on the hull area below the waterline — called the Wetted Surface Area (WSA).
Dellenbaugh Formula (simplified): WSA = LWL × (1.7 × Draft + 0.5 × Beam)
Where all measurements are in feet and WSA is in square feet.
This formula is a practical approximation used by marine painters. Actual WSA varies by hull form — a full-keel sailboat has more WSA than a flat-bottom powerboat of the same length.
Paint calculation: Gallons needed = (WSA / Coverage Rate) × Number of Coats
Typical coverage rates:
- Most antifouling paints: 350–450 sq ft per gallon (listed on can)
- High-build paints: 250–350 sq ft per gallon
- Ablative/self-polishing paints: 300–400 sq ft per gallon
Number of coats:
- New boat (bare bottom): 3 coats minimum
- Recoat over existing paint (good condition): 1–2 coats
- Recoat over worn paint: 2–3 coats
Antifouling paint types:
- Hard (non-ablative): Does not wear off; builds up over seasons; good for fast boats.
- Ablative (self-polishing): Wears away slowly, always exposing fresh biocide; better for slower boats left in water year-round.
- Hybrid: Combines properties of both types.
Tips:
- Add 10–15% overage for waste, drips, and second-pass touch-ups.
- Apply in 2 directions (horizontal then vertical) for better coverage.
- Most paints require application within a specific temperature and humidity range.
- Roll the paint on with a 3/8 nap roller; use a brush for corners and keels.