How much paint do I need?
The amount of paint a room needs comes down to the wall area, how many coats you're applying, and the paint's coverage (how far a gallon or liter stretches). Measure the room, subtract the doors and windows you won't be painting, multiply by the number of coats, and divide by coverage. This calculator does all of that and rounds up to whole cans so you don't run out halfway through the second coat.
How to use this calculator
- Choose Imperial (feet, gallons) or Metric (meters, liters).
- Enter the room's length, width, and wall height.
- Enter how many doors and windows to leave unpainted.
- Set the number of coats and adjust coverage if your paint's label differs from the default.
The paint formula
Wall area = perimeter × height, where perimeter = 2 × (length + width). Subtract the openings (about 21 sq ft per door and 15 sq ft per window), multiply by coats, and divide by coverage.
Worked example: a 12 ft × 12 ft room with 9 ft walls, one door, two windows, two coats. Perimeter = 48 ft, wall area = 432 sq ft. Subtract 21 + (2 × 15) = 51 sq ft → 381 sq ft paintable. Two coats = 762 sq ft ÷ 350 = 2.2 gallons, so you'd buy 3 gallons.
What affects paint coverage
- Surface texture: smooth, sealed walls reach 350–400 sq ft per gallon; rough or textured surfaces use more.
- Porosity: bare drywall, patched areas, and fresh plaster soak up paint — prime them first.
- Color change: going from dark to light (or vice versa) may need an extra coat.
When in doubt, buy a little extra from the same batch so touch-ups match later on.