Calculate Paint Coverage

This calculator estimates how much wall paint you need. Enter the total length of the walls, their height, the area of doors and windows, the coverage printed on the can and the number of coats, and you get the paintable area, the gallons and litres and the whole cans to buy.

Formula

Paintable area A = wall length × height − openings. Paint = A × coats ÷ coverage. Litres = US gallons × 3.785411784.

• l: the sum of all wall lengths (the room perimeter) in feet
• h: wall height in feet
• openings: doors and windows in square feet (a standard door is roughly 20 sq ft)
• coverage: square feet one gallon covers, from the label (metric labels give m² per litre; 1 m²/L ≈ 40.7 sq ft/gal)

Worked Example

A 12 ft × 12 ft room (48 ft of wall), 8 ft high, 40 sq ft of doors and windows, a label coverage of 350 sq ft per gallon, 2 coats.

1. 48 × 8 = 384; 384 − 40 = 344 sq ft
2. 344 × 2 ÷ 350 = 1.97 gal
3. 1.9657 × 3.7854 = 7.44 L
4. Round up: 2 cans

Limits and common mistakes

• Rough, porous or dark-to-light surfaces take more paint than the label says; primer has its own coverage.
• Ceilings are not included: add length × width of the room as a separate area.
• Measure in one unit. For metres, convert the result: 1 m² = 10.764 sq ft.