MEASURE. ESTIMATE. BUILD.
Estimate exterior paint by wall area.
Change any measurement to update your estimate.
Enter your home’s wall dimensions to get gallons for two coats, with a slightly higher waste factor for rougher siding.
Exterior siding is rougher and thirstier than interior drywall, so this view leans on a slightly higher waste factor for two coats. Measure wall area minus the big openings, and plan around weather — most paints need dry, mild conditions to cure.
Always calculate for the coats you'll actually apply — nearly every color change needs two, and primer is a separate gallon on bare drywall or a big color jump. The 350 ft²/gallon spread rate assumes primed, smooth walls; textured or porous surfaces drink more, so round up. Box your paint (combine cans into one bucket) so the color stays consistent wall to wall.
The math behind the tool:
gallons = wall area ÷ 350 × coats × (1 + waste%)Sources: U.S. EPA — Indoor Air Quality (low-VOC interior paint and indoor air quality).
Multiply the room perimeter by the wall height for total area, subtract doors and windows, then divide by 350 ft² per gallon and multiply by the number of coats. A 12×12 room usually needs about 2 gallons for two coats.
One gallon of interior paint covers about 350–400 ft² in a single coat on primed, smooth walls; textured or porous surfaces cover less.