Paint Calculator
Enter your room size, how many coats you're applying, and how many doors and windows to skip — the calculator works out how much paint to buy.
Buy too little paint and you're back at the store mid-project trying to match the batch; buy too much and it dries out in the garage. This calculator estimates the wall area, subtracts doors and windows, multiplies by your number of coats, and divides by a standard coverage rate to tell you how much to buy.
How to calculate how much paint you need
Find the total wall area (the perimeter times the wall height), subtract the doors and windows you won't paint, multiply by the number of coats, and divide by the paint's coverage per gallon.
Paintable area = Wall area − doors − windows
Paint = (Paintable area × coats) ÷ coverage per gallon
| Assumption | Value used |
|---|---|
| Coverage per gallon | ≈ 350 sq ft (32 m² per litre band) |
| Area per door | ≈ 21 sq ft (1.95 m²) |
| Area per window | ≈ 15 sq ft (1.4 m²) |
Worked example
A 12 ft × 12 ft room with 8 ft walls, two coats, one door and two windows:
| Wall area = 2 × (12 + 12) × 8 | 384 sq ft |
| Less 1 door + 2 windows (21 + 30) | 333 sq ft |
| × 2 coats = 666 sq ft to cover | ÷ 350 |
| Paint needed | 1.9 → 2 gallons |
How much does a gallon of paint cover?
Most interior wall paints cover about 350 to 400 square feet per gallon on a smooth, previously painted surface. Rough, porous or brand-new drywall soaks up more, so coverage drops and a primer coat is worth adding. Dark-to-light colour changes usually need an extra coat too.
Tips to avoid running out
Round up to whole cans — this calculator does, since you can't buy a fraction of a gallon. Keep a little left over for touch-ups, and if you're covering a bold colour or bare drywall, add a primer coat to the count. Buying all your paint in one batch avoids slight colour differences between production runs.