Area to paint
365 ft²
Paint Calculator
How many gallons a room actually needs — doors and windows taken off, and the second coat counted.
Floor to ceiling.
21 sq ft taken off each.
15 sq ft taken off each.
Square feet, one coat. It's on the can — usually 350 to 400.
3 gallons to buy
365 sq ft, 2 coats. You'll have some left over.
Paint needed
2.09 gal
Estimated cost
—
Taken off
51 ft²
Coverage is the number that moves. A gallon covers 350 to 400 sq ft
on smooth, sealed drywall. On bare drywall, textured walls, or rough render it can
drop to 250 or less, because the surface drinks it. The figure is printed on the can
— use that one, and knock it down if the wall is porous.
How this is worked out
Walls
(length + width) × 2 × height = wall area
That is the perimeter times the height. Then take off the openings:
wall area − (doors × 21) − (windows × 15) = area to paint
Ceiling
length × width = ceiling area
Then, either way
area × coats ÷ coverage per gallon = gallons, rounded up to whole cans.
Worked example — a 14 × 12 ft room, 8 ft walls, 2 coats
- Perimeter:
(14 + 12) × 2 = 52 ft - Wall area:
52 × 8 = 416 sq ft - One door and two windows:
21 + 30 = 51 sq ft off - Area to paint:
416 − 51 = 365 sq ft - Two coats:
365 × 2 = 730 sq ft of coverage needed - At 350 per gallon:
730 ÷ 350 = 2.09, so 3 gallons
Standard sizes used here
| Opening | Area deducted |
|---|---|
| Door | 21 sq ft (3 ft × 7 ft) |
| Window | 15 sq ft (3 ft × 5 ft) |
If your windows are much bigger or smaller, the quickest fix is to change the window count rather than the maths — two standard windows instead of one big picture window, for instance.
Questions people ask
Should I really deduct doors and windows?
For one or two openings in a normal room, it barely changes the answer and you may as well not bother — the rounding up to whole gallons swallows it. It matters when you have a wall that is mostly glass, or a whole house to do, where ignoring openings can put you a full gallon out.
Why does it always say three gallons when I need 2.1?
Because paint is sold in whole gallons and running out in the middle of the second coat is worse than having some left. If you are close to a round number, the usual trick is to buy two gallons and a quart rather than three.
Do I need primer as well?
Over bare drywall, patched areas, or a dark colour you are covering with a light one, yes. Primer covers about the same area per gallon, so run the numbers again with one coat to work out how much. Self-priming paint is really just paint that expects you to do two coats anyway.
What about trim, doors and the ceiling?
Trim is usually sold and bought by the quart because the area is small. A standard door takes about a quart for both sides. Switch the calculator to ceiling to do the ceiling on its own — it is nearly always a different paint.
How much does a second coat really add?
Close to the same again. A second coat does not go further than the first on a properly covered wall. The only time it uses less is over a sealed, same-colour base, and even then plan on 70 to 80%.