Boat Paint Calculator
A 24 ft powerboat with an 8.5 ft beam needs about 0.8 gallons of antifouling bottom paint for two coats. Enter your boat to size bottom paint, barrier coat, topside enamel, primer, or bilge paint.
- Free, no signup
- Bottom, topside, primer, bilge
- Copper-free picks for aluminum
- Gallons and cans to buy
How much bottom paint by boat length
Antifouling bottom paint for two coats, by boat length and a typical beam for that size. Use the planing column for most powerboats and fin-keel sailboats, and the displacement column for trawlers and full-displacement hulls.
| Boat length | Typical beam | Bottom area, planing | Planing or fin keel | Displacement or trawler |
|---|---|---|---|---|
| 16 ft | 6.5 ft | 73 sq ft | 0.40 gal, buy 1 | 0.51 gal, buy 1 |
| 20 ft | 8 ft | 112 sq ft | 0.62 gal, buy 1 | 0.79 gal, buy 1 |
| 24 ft | 8.5 ft | 143 sq ft | 0.79 gal, buy 1 | 1.01 gal, buy 2 |
| 28 ft | 9.5 ft | 186 sq ft | 1.02 gal, buy 2 | 1.32 gal, buy 2 |
| 32 ft | 11 ft | 246 sq ft | 1.36 gal, buy 2 | 1.74 gal, buy 2 |
| 36 ft | 12.5 ft | 315 sq ft | 1.73 gal, buy 2 | 2.23 gal, buy 3 |
| 40 ft | 14 ft | 392 sq ft | 2.16 gal, buy 3 | 2.77 gal, buy 3 |
Assumes 2 coats at 400 sq ft per gallon per coat, plus 10% waste. Bottom area is length overall x beam x 0.70 for planing and fin-keel hulls, or x 0.90 for displacement hulls. The buy figure rounds up to whole gallons; the calculator also prices gallon and quart mixes. Pontoons: a 22 ft two-tube pontoon needs about 0.79 gallons of copper-free antifouling for two coats, and a 24 ft tritoon about 1.29 gallons.
How the boat paint calculator works
For bottom jobs the calculator estimates the painted area from length and beam with a hull factor. The industry rule is waterline length x beam x 0.75 to 0.77 for planing and fin-keel hulls. Waterline runs about 93 percent of overall length on most boats, so the calculator applies 0.70 to length overall. Gallons are area x coats, divided by the paint's coverage rate, plus 10 percent waste.
- Antifouling bottom paint. 400 sq ft per gallon per coat. Two coats is the standard system, with a third pass on the waterline and leading edges.
- Barrier coat. 250 sq ft per gallon. Epoxy barrier coat goes on before antifouling on a bare, stripped, or blistered hull, and a new barrier system takes 4 to 5 coats.
- Topside enamel and bilge paint. 350 sq ft per gallon. Measure the actual surfaces and use the area option: topsides, transom, and deck areas vary too much to estimate from length alone.
- Primer. 300 sq ft per gallon, usually one coat over a cleaned, sanded surface.
- Pontoons. The painted area is the tube surface down to the waterline (about half the girth of a 25 in tube), so beam is ignored. Aluminum tubes get copper-free antifouling only.
Boat paint FAQ
How much bottom paint do I need for a 30 foot boat?
A typical 30 ft planing powerboat with a 10 ft beam has about 210 sq ft of bottom. Two coats of antifouling at 400 sq ft per gallon works out to roughly 1.2 gallons, so buy 2 gallons, or a gallon and a quart. Run your own numbers above for an exact estimate.
How many coats of antifouling paint should I apply?
Two full coats is the standard system for ablative and hard antifouling, with an extra pass on the waterline, leading edges, and rudder where the paint wears fastest. One coat rarely lasts a season.
How much area does a gallon of boat paint cover?
The calculator uses 400 sq ft per gallon per coat for antifouling, 250 for barrier coat, 350 for topside enamel and bilge paint, and 300 for primer. The can label gives the exact figure for each product, and rough or bare surfaces take more paint.
What bottom paint is safe for a pontoon boat?
Aluminum tubes need copper-free antifouling. Copper-based paint in contact with aluminum sets up galvanic corrosion that eats the tubes. Pick the pontoon hull option in the calculator and it only recommends copper-free formulas.
How do I measure the bottom area of my boat?
The standard estimate is waterline length times beam times a hull factor: about 0.75 to 0.77 for planing hulls and fin-keel sailboats, and up to 0.9 for full-displacement hulls. For a precise number, measure your hull below the waterline in sections.
Does the estimate include a waste factor?
Yes. The calculator adds 10 percent for roller and tray losses, overlap, and touch-up. Coverage also varies by surface roughness, so a first coat over bare or sanded gelcoat runs heavier than the numbers on the can.