Estimate paint quantities and costs for your boat project.
10-15% for brush/roller, 20-30% for spray.
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Download NowBuy too little antifouling and you stop mid-spring to chase a quart; buy too much and you store expensive, hazardous leftovers. The right quantity comes from the real surface area, the number of coats each product needs and the coverage rate printed on the tin, not from a glance at the hull.
Different coatings spread very differently: primer and antifouling are thicker and cover fewer square feet per gallon than a hard topcoat or interior enamel, so a mixed job has to be tallied product by product before you add anything to the basket.
Add each surface with its area, paint type and coat count, set a waste allowance, and the calculator groups by product, applies each coverage rate, and rounds to the nearest quart so you know exactly how many gallons to buy and what they will cost.
Gallons = ceil(Area × Coats × (1 + Waste) ÷ Coverage × 4) ÷ 4| Paint Type | Coverage (ft²/gal) | Coats |
|---|---|---|
| Primer | 350 | 1 - 2 |
| Topcoat / enamel | 400 | 2 |
| Antifouling | 300 | 2 - 3 |
| Interior | 450 | 2 |
Typical coverage and use by marine paint type.
Formula verified against standard naval-architecture references · Updated October 2026
Multiply each surface's area by the number of coats and a waste allowance, then divide by the product's coverage rate in square feet per gallon, and round up to quarts. Because primer, topcoat, antifouling and interior paint all spread differently, work them out one product at a time. This calculator groups your surfaces by paint type so the per-product totals and the overall cost come out in a single pass.
Most foul-release and ablative antifoulings want two to three full coats over a sound primed bottom, and the manufacturer's data sheet gives the correct count and the recoat window. Under-coating shortens the effective season and can leave bare patches that gall quickly, whereas a couple of thin, well-spaced coats beat one thick one that stays soft. Always respect the minimum intervals between coats.
Around 10 percent for smooth, brushed or rolled surfaces, and 15 to 20 percent for spraying, rough fairing, or awkward geometry where overspray and drip are unavoidable. The allowance also absorbs the paint that never leaves the tray, brush and lines. First-time painters and heavily profiled hulls should budget toward the higher end rather than hoping for the best.
Because their solids, viscosity and purpose differ. Antifouling and primer carry heavy biocide and pigment solids and lay down thick, so they cover fewer square feet per gallon. Topcoats and interior enamels are applied thinner for finish, so they spread further. Never assume one product's coverage applies to another; use the figure on the specific tin for each coat.
Yes, and add the boot-topping and any turned-up areas you actually coat. A quick approximation is roughly the length times the sum of the beam and twice the draft, adjusted for flare, but a real bottom job needs the paintable area, so measure carefully along the keel and around the appendages. Under-measuring the bottom is the usual reason builders run short of antifouling.
Indirectly, through coverage and pot life. Cold, thick paint spreads less per gallon and may need thinning, while hot, thin paint runs and forces extra coats. Spray conditions also change transfer efficiency. None of that alters the arithmetic of this tool, but it is why builders often need a little more than the textbook figure in extreme weather, so keep some material on hand.
The spread rate on the tin assumes one smooth coat over a primed surface brushed or rolled by an experienced hand at a moderate temperature. Reality eats that figure: a rough, porous or freshly faired hull soaks up the first coat, hot weather and thin mixtures spread thinner but flash off before you can lay them down, and back-rolling or spraying wastes more than a careful brush pass. Add a generous ten to twenty percent to the calculated volume, and when you are down to the last gallon on a big job, do not stretch it with solvent, because a starved coat peels early and costs far more to redo than the paint you thought you saved.