The formula
perimeter = 2 × (length + width)
gross wall = perimeter × ceiling height
net wall = gross − doors − windows
gallons = (net × coats) ÷ coverage
gross wall = perimeter × ceiling height
net wall = gross − doors − windows
gallons = (net × coats) ÷ coverage
Standard deductions: 21 ft² per door (3 × 7 ft) and 15 ft² per window (3 × 5 ft).
Worked example: a 12 × 12 room
Eight-foot ceilings, one door, two windows, two coats, 350 ft² per gallon:
| Step | Working | Result |
|---|---|---|
| Perimeter | 2 × (12 + 12) | 48 ft |
| Gross wall area | 48 × 8 | 384 ft² |
| Less door + 2 windows | 21 + 30 | −51 ft² |
| Net wall area | 384 − 51 | 333 ft² |
| Two coats | 333 × 2 | 666 ft² |
| Paint needed | 666 ÷ 350 | 1.9 → 2 gallons |
333
ft² of wall
2 gal
walls, 2 coats
1 gal
ceiling, 2 coats
Enter your room and get gallons or litres.Paint calculator →
Common room sizes
Two coats on walls, one door, standard windows, 350 ft² per gallon:
| Room | Ceiling | Net wall | Gallons |
|---|---|---|---|
| 10 × 12 ft | 8 ft | 316 ft² | 2 |
| 12 × 12 ft | 8 ft | 333 ft² | 2 |
| 12 × 16 ft | 8 ft | 397 ft² | 2.5 |
| 15 × 20 ft | 9 ft | 564 ft² | 3.5 → 4 |
Round up to whole gallons for the first coat, and consider quarts for the remainder. Two gallons plus a quart is often cheaper and less wasteful than three gallons — and you will want a little left over for touch-ups anyway.
When coverage drops
The 350 ft² figure assumes smooth, primed, previously painted drywall. Lower it when:
- Bare drywall or new plaster — porous, absorbs heavily. Prime first, or expect 250 ft² per gallon.
- Textured walls — knockdown or orange-peel texture adds surface area. Cut coverage 25% to 30%.
- Masonry, brick or concrete block — can halve coverage.
- Dark to light colour changes — budget for a primer coat plus two finish coats.
Buy all the paint for a room in one trip, and check the batch numbers match. Colour is mixed in batches and small variations between them are visible on a large wall in daylight.
Do not forget the extras
Trim, doors and window frames use a different sheen and are bought separately — a quart typically covers the trim in an average room. Ceilings use flat paint: at 144 ft² for a 12 × 12 room, two coats is 288 ft², comfortably under a gallon.
Estimating a larger project? Concrete works the same way — measure the volume, add an allowance for waste, then convert to what the store actually sells.
Run your own numbers
FAQ
How much does a gallon of paint cover?
About 350 to 400 square feet per coat on smooth, primed walls. Rough, porous or previously unpainted surfaces absorb 25% to 30% more, so plan on 250 to 300 square feet per gallon for bare drywall, masonry or textured finishes.
How much paint for a 12×12 room?
About 2 gallons for two coats on the walls. The perimeter is 48 feet, so at an 8-foot ceiling that is 384 square feet of gross wall, less roughly 51 square feet for a door and two windows — 333 square feet net, or 666 square feet across two coats.
Do I need two coats of paint?
Usually yes. One coat rarely produces even colour depth, especially over a different shade. Two coats is standard; going from dark to light, or covering bare drywall, often needs a primer plus two coats.
How do I calculate wall area?
Add the length of all walls to get the perimeter, multiply by ceiling height for the gross area, then subtract 21 square feet per standard door and about 15 square feet per average window.
Should I paint the ceiling too?
If you are painting it, add the room's floor area as a separate figure. A 12×12 ceiling is 144 square feet, so two coats is 288 square feet — well under one gallon. Ceilings use flat paint, so buy it separately rather than stretching the wall paint.