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.
Field notes
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.
How the exterior paint calculator works
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).