Paint Calculator
Enter your wall dimensions, the number of doors and windows to exclude, and how many coats you're applying, to find exactly how many gallons of paint to buy.
How the Paint formula works
Paintable area, adjusted for openings, divided by coverage:
Wall area = Perimeter × Wall height Paintable area = Wall area − (Doors × 20 ft²) − (Windows × 15 ft²) Gallons needed = (Paintable area × Coats) / Coverage per gallon
Coverage defaults to 350 ft² per gallon, a standard mid-range estimate for interior wall paint.
Step-by-step calculation
- Multiply the room's perimeter by the wall height to get total wall area.
- Subtract standard area estimates for each door (20 ft²) and window (15 ft²).
- Multiply the paintable area by the number of coats you plan to apply.
- Divide by the paint's coverage rate (default 350 ft²/gallon) to get gallons needed.
Worked example
A room with a 56 ft perimeter, 9 ft ceilings, 1 door, and 2 windows, painted with 2 coats: Wall area = 56 × 9 = 504 ft². Paintable area = 504 − 20 − 30 = 454 ft². Total coverage needed = 454 × 2 = 908 ft². Gallons = 908 / 350 ≈ 2.6, so plan to buy 3 gallons.
Frequently asked questions
Should I round up or down on the gallons needed?
Always round up — paint is sold in fixed can sizes, and running short mid-project means a mismatched-batch touch-up later, so it's better to have a little extra.
Does this include the ceiling?
No — this calculator covers wall area only. For ceilings, calculate the room's square footage separately (length × width) and divide by the same coverage rate.
How many coats do I actually need?
Two coats is standard for most interior repaints, especially over a different color. A single coat can work for touch-ups or when painting the exact same color, and dramatic color changes sometimes need a primer coat first.