Free estimator
Paint Calculator
Use this paint calculator to estimate wall area, paintable area, gallons to buy, and optional paint cost for a room.
Last updated: June 23, 2026
How to measure
Measure room length, width, and wall height. The calculator uses the room perimeter multiplied by wall height, then subtracts estimated door and window area before applying coats and coverage.
Formula
wall area = room perimeter x wall height
paintable area = wall area - doors area - windows area
gallons = paintable area x coats / coverage per gallon
Tips before buying
- Check the coverage on your paint label before buying.
- Plan extra paint for repairs, texture, dark color changes, and future touch-ups.
- Primer may reduce the amount of finish paint needed on patched or porous surfaces.
FAQ
How much area does one gallon of paint cover?
Many interior paints cover about 350 to 400 square feet per gallon, but the exact coverage depends on the product, surface, color change, and application method.
Should I count doors and windows?
Yes. This calculator subtracts common estimates of 21 sq ft per door and 15 sq ft per window to avoid overestimating wall paint.
Do I need two coats?
Two coats are common for interior walls, especially with a color change. Primer, wall texture, and paint quality can change the final amount.