Quick answer: add up the wall area (room perimeter × wall height), subtract about 21 sq ft per door and 15 sq ft per window, multiply by the number of coats (usually 2), and divide by the coverage on the can, about 350–400 sq ft per gallon (or 10–12 m² per litre). A typical 12 × 12 ft bedroom needs about 2 gallons for two coats on the walls. Try the paint calculator.
1. Measure the walls
- Measure the length of each wall and add them up to get the perimeter. For a rectangular room, that’s 2 × (length + width).
- Measure the wall height from floor to ceiling.
- Wall area = perimeter × height.
Example: 12 × 12 ft room, 8 ft high → 2 × (12 + 12) × 8 = 384 sq ft.
2. Subtract doors and windows
| Opening | Typical area |
|---|---|
| Standard door | about 21 sq ft (1.9 m²) |
| Standard window | about 15 sq ft (1.5 m²) |
| Patio door or large window | measure it |
384 − 21 (door) − 15 (window) = 348 sq ft.
3. Multiply by coats
Plan on two coats for most rooms. You may need three when:
- going from a dark colour to a light one (or using a deep, saturated colour)
- painting new drywall or fresh plaster (start with a primer or mist coat)
- covering stains, patches or water marks (spot-prime first)
348 × 2 coats = 696 sq ft of coverage.
4. Divide by coverage
Most interior wall paint covers about 350–400 sq ft per gallon (10–12 m² per litre) per coat on smooth, primed walls. Rough, textured or porous surfaces can drop that by a third or more. Use the figure on your can.
696 ÷ 375 ≈ 1.9 gallons, so buy 2 gallons.
Worked examples
| Room | Paint for walls (2 coats) | Suggested cans |
|---|---|---|
| 12 × 12 ft bedroom, 8 ft high, 1 door, 1 window | about 1.9 gal | 2 × 1 gal |
| Same room, plus the ceiling | about 2.6 gal | 2 × 1 gal + 3 quarts (or buy ceiling paint separately) |
| 4 × 3 m room, 2.4 m high, 1 door, 1 window | about 6 L | 1 × 5 L + 2 × 1 L |
| 5 × 4 m room, 2.4 m high, 1 door, 2 windows, plus ceiling | about 11.7 L | 1 × 10 L + 2 × 1 L |
These figures come from our paint calculator, which also picks the can sizes that leave the least paint over.
Ceilings, trim and primer
- Ceiling area = room length × width. Most people use a separate flat white ceiling paint, so work it out on its own.
- Trim, doors and baseboards use a different (usually semi-gloss) paint. For one room, a quart (about 1 L) of trim paint is often enough.
- Primer coverage is similar to paint. Use it on new drywall, bare wood, glossy surfaces and big colour changes.
Tips so you don’t run short
- Buy enough of the same colour mix at once. Tinted batches can vary slightly, so if you need more than one gallon, mix (“box”) the cans together in a bucket before you start.
- Keep a little spare for touch-ups, labelled with the room and colour code.
- Two thin coats beat one thick one. They dry evenly and cover better.
- Feature wall? Measure it separately and subtract it from the main colour.
FAQ
How much paint do I need for a 12×12 room?
About 2 gallons for two coats on the walls of a 12 × 12 ft room with 8 ft ceilings, one door and one window. Add about 0.8 gallon if you’re also painting the ceiling.
How many square feet does a gallon of paint cover?
About 350–400 sq ft per coat on smooth walls. Textured or unprimed walls take more.
How much paint do I need for one wall?
Multiply the wall’s width × height, subtract any door or window, and multiply by two coats. A 12 × 8 ft feature wall (96 sq ft) needs about half a gallon for two coats, so a quart won’t be enough; buy a gallon.
Do I really need two coats?
Usually yes. One coat often looks patchy, especially with colour changes or on fresh patches. Some “paint and primer” products cover in one coat over a similar colour.
How this guide was written: coverage and opening sizes are typical figures from paint manufacturers. Always check the coverage printed on your can. See how I test and review.