Paint Calculator
Work out how much paint a room needs, with doors and windows deducted and coats accounted for properly. Enter the coverage from your tin and get the number of cans to buy.
Switching units keeps your measurements. Nothing is reset.
What are you painting?
Doors and windows
No openings yet. Without any, the full wall area is painted.
Your paint
Area one gal covers, printed on the tin.
Optional. Add it to get whole cans rather than a volume.
Estimated cost
Cost uses the price you enter. This product never shows invented market prices.
Your result appears here
Enter your measurements to see how much material to buy.
How to use this calculator
- 1
For a whole room, enter the floor length and width and the wall height. The calculator works out the band of wall around the room, so there is no need to measure four walls separately.
- 2
Tick the ceiling box if you are painting that too, or add a ceiling on its own if the walls are staying as they are.
- 3
Add each door and window with a count. Three identical windows is one row with a count of three.
- 4
Choose the number of coats, then enter the coverage printed on your tin. Add the container size if you want a can count rather than a volume.
A worked example
A 12 ft × 15 ft room with 8 ft walls, one door and two windows, two coats, paint covering 350 ft² per gallon, bought in one gallon cans:
Wall and ceiling area
432 ft²432 ft²
Deduct the openings
432 ft² − 44.25 ft²387.8 ft²
Multiply by the coats
387.8 ft² × 2775.5 ft²
Divide by the coverage
775.5 ft² ÷ 350 ft²2.22 gal
Divide by the coverage of one package
2.22 gal ÷ 1 gal2.22
Round up to whole packages
round up 2.223
Coverage you are buying
3 × 1 gal3 gal
You will need
3 cans
This example is produced by running the calculator above, so it always matches what the tool does.
The formula
Wall area is the room perimeter, 2 × (length + width), multiplied by the wall height. Doors and windows are subtracted from that to give the surface to paint. That surface is then multiplied by the number of coats, which gives the total area to cover — a larger number than the wall itself, because you are painting it more than once. Dividing by the coverage on the tin gives the volume of paint, and dividing that by the container size and rounding up gives the cans to buy.
Getting a realistic figure
Coverage on the tin is a best case
The figure quoted assumes a smooth, sealed, similar-coloured surface. Bare plaster, textured walls and a strong colour change all reduce it, sometimes by a third. If in doubt, use the lower end of any range printed on the tin.
Deduct big openings, not every one
A patio door is worth deducting. A single small window in a large room makes little difference and deducting it can leave you just short. The calculator lets you choose either way.
Two coats is normal, one is optimistic
Most repaints need two coats for an even finish, and a light colour over a dark one often needs three or a separate primer. Primer is a different product with its own coverage, so calculate it as a separate job.
Ceilings and trim are different products
Ceiling paint and trim paint have their own coverage figures. Run the ceiling as its own surface with its own tin figure rather than lumping it in with the walls.
Common questions
- How much paint do I need for a 12x15 room?
- With 8 ft walls that is 432 square feet of wall. Take off a door and two windows and you have about 388 square feet. Two coats is 776 square feet of painting, which at 350 square feet per gallon is about 2.2 gallons, so three one-gallon cans.
- Does a second coat need as much paint as the first?
- Roughly, on a sealed surface. This calculator multiplies the surface area by the number of coats, which is the standard assumption. On bare plaster the first coat soaks in and uses noticeably more, so treat that case as needing extra.
- Should I subtract doors and windows?
- For large openings, yes. For a couple of small windows, many decorators do not bother, because the saving is smaller than the natural variation in coverage. Both approaches are available above.
- How many square feet does a gallon of paint cover?
- It depends entirely on the product, which is why this calculator asks rather than assuming. The figure is printed on the tin, usually as a range, and the real result also depends on the surface you are painting.