This view estimates a single room and subtracts a standard door and window before applying two coats. Textured walls drink more than the 350 ft²/gallon assumption, so round up and box your cans for consistent color.
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 paint for a room 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).