Paint Calculator
Enter the room size, doors, windows and coats to see how many gallons of paint to buy.
Quick answer
2 gallons for 302 ft² of wall (2 coats)
Leftover paint stays in the can, so no waste is added.
2 gallons for 302 ft² of wall (2 coats)
Your measurements
- Measure by
- Room length, width and wall height
- Length
- 12 ft
- Width
- 10 ft
- Wall height
- 8 ft
- Doors
- 1
- Windows
- 2
- Door size
- 20 ft²
- Window size
- 15 ft²
- Coats
- 2
- Coverage
- 350 ft² per gallon
- Paint the ceiling
- No
- Add primer (1 coat)
- No
Buy list
Wall paint
2 gallons of wall paint
1.73 gallons exact
Add your local prices to see a total.
Project figures
- Walls
- 302 ft²: 1.73 gal (6.53 L) exact
Diagram
Not to scale where a dimension is very thin or long.
Show the math
- Wall area = 2 × (length + width) × wall height − doors − windows = 302 ft².
- Wall paint = 302 ft² × 2 coats ÷ coverage = 1.73 gallons, rounded up to 2.
Assumptions and notes
- Coverage varies with the paint and the surface; rough or porous walls need more.
- Ceilings usually take a different paint, so they are a separate line.
- Doors and windows are left unpainted at the sizes entered.
- Quantities and cost only. For structural, load or code questions, check your local building code and a qualified professional.
How it works
This calculator works out how much paint a room needs: the wall area left after the doors and windows, the gallons for the number of coats you plan, and, if you want them, separate amounts for the ceiling and for a coat of primer. Each one becomes its own line on the buy list in whole gallons, because that is how paint is sold.
The formula
The walls of a rectangular room, laid out flat, make one long strip: the perimeter, which is twice the length plus twice the width, times the wall height. From that the calculator subtracts each door and window. It assumes a door of about 20 square feet and a window of about 15 square feet, and you can change both sizes. The paint needed is the remaining area times the number of coats, divided by the coverage printed on the can. If the room is not a simple rectangle, switch to total wall area and type the area you measured.
A worked example
The page opens with a 12 ft × 10 ft room with 8 ft walls, one door and two windows. The perimeter is 44 ft, so the walls cover 44 × 8 = 352 square feet. Taking away one door and two windows leaves 302 square feet. Two coats at 350 square feet per gallon need 302 × 2 ÷ 350 = 1.73 gallons, so you buy 2. Painting the ceiling too adds 120 square feet × 2 coats, or 0.69 gallons: one more gallon of ceiling paint.
Coats, coverage and primer
Most finishes look even after 2 coats, which is the default. Coverage depends on the paint and the wall: smooth, previously painted walls reach the figure on the can, while new plaster, bare drywall and rough textures soak up more. Primer is one coat over everything that gets painted, walls and ceiling together, at 350 square feet per gallon unless you change it. High-build primers cover much less, so check the label before relying on the default.
Ceilings, and when there is nothing to paint
Ceilings usually take a flat white paint that differs from the walls, so the ceiling is a separate line. If you enter more door and window area than wall, the calculator says so instead of giving a negative amount. If the openings exactly match the walls, it tells you there is nothing to paint rather than suggesting zero gallons.
Measuring tips and common mistakes
- Measure wall height from the floor to the ceiling, not to the top of the trim.
- Count closet doors and large openings; leave small switches and outlets in.
- Buying for one coat when the colour change needs two: dark to light usually needs more.
- Forgetting that the second gallon of the same colour should come from the same batch.