Paint Calculator for Walls (Gallons or Litres)
A 12 × 12 ft room with 8 ft walls, one door and two windows needs about 1.9 gallons for two coats, so buy 2 gallons. The calculation is wall area minus doors and windows, times the number of coats, divided by the coverage on the paint can (often 350–400 sq ft per gallon).
Worked example: a 12 × 12 ft bedroom
The room is 12 ft by 12 ft with 8 ft walls, one door and two windows. You want two coats of a paint rated at 350 sq ft per gallon.
- Perimeter: 12 + 12 + 12 + 12 = 48 ft.
- Wall area: 48 × 8 = 384 sq ft.
- Subtract openings: 384 − 21 (door) − 2 × 15 (windows) = 333 sq ft.
- Two coats: 333 × 2 = 666 sq ft of coverage.
- Paint: 666 ÷ 350 = 1.9 gallons, so buy 2 gallons.
Metric version: a 4 × 3 m room with 2.4 m walls has 14 × 2.4 = 33.6 m², minus 1.9 m² and 1.4 m² = 30.3 m². Two coats at 8.6 m² per litre need about 7.05 litres, so three 2.5 L tins (7.5 L).
How the paint calculation works
paint = (perimeter × height − doors × door area − windows × window area) × coats ÷ coverage
- Coverage is the area one gallon (or litre) covers in one coat. The default, 350 sq ft per gallon (about 8.6 m² per litre), is at the low end of typical interior wall paint ratings, which gives a little margin. Use the number on your can.
- Door area is assumed to be 21 sq ft (3 × 7 ft, about 1.9 m²) and window area 15 sq ft (3 × 5 ft, about 1.4 m²).
- Limitations: ceilings, trim and primer are not included. Textured or porous surfaces use more paint.
- Safety: homes built before 1978 may have lead-based paint. Follow EPA guidance before sanding or scraping old paint.
Paint needed by room size (8 ft walls, 1 door, 2 windows, 2 coats, 350 sq ft/gal)
| Room size | Wall area to paint | Gallons needed | Buy |
|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.5 | 2 gal |
| 12 × 12 ft | 333 sq ft | 1.9 | 2 gal |
| 12 × 15 ft | 381 sq ft | 2.2 | 3 gal |
| 15 × 15 ft | 429 sq ft | 2.5 | 3 gal |
| 15 × 20 ft | 509 sq ft | 2.9 | 3 gal |
| 20 × 20 ft | 589 sq ft | 3.4 | 4 gal |
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and two windows, a 12 × 12 ft room has about 333 sq ft of wall to paint. Two coats at 350 sq ft per gallon need about 1.9 gallons, so 2 gallons is enough.
How many square feet does a gallon of paint cover?
Many interior wall paints are rated for roughly 350 to 400 square feet per gallon per coat on smooth surfaces. The exact figure is printed on the can and varies by product and surface.
Do I really need two coats of paint?
Two coats are usual when changing colour or painting new drywall, because one coat often looks patchy. A single coat can be enough when repainting the same colour with a high-hiding paint.
Should I subtract doors and windows?
Yes. This calculator subtracts about 21 sq ft (1.9 m²) per door and 15 sq ft (1.4 m²) per window. If your openings are much larger, measure them and reduce the perimeter or count extra windows.
Does rough or porous surface change how much paint I need?
Yes. Textured walls, bare plaster and new drywall absorb more paint, so coverage is lower than the can's rating. Lower the coverage value in the calculator, or use a primer first.
Related calculators
Sources
- Paint Calculator — Benjamin Moore
- Paint Calculator — PPG Paints
- Paint Calculator — Dulux UK
- Protect Your Family from Sources of Lead — U.S. Environmental Protection Agency (EPA)
Disclaimer: results are estimates for general guidance only. Check product labels, recipes and project instructions before relying on them.