Boat Bottom Paint Calculator

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, and a deep fin keel adds two painted faces the formula only half accounts for.

Paint calculation: Gallons needed = (WSA / Coverage Rate) × Number of Coats

The calculator adds 12% on top for waste, drips, roller absorption and the second pass you always end up doing on the leading edge of the keel. That is the middle of the 10 to 15% painters normally allow.

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:

• 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.