How to use the paint calculator
- Measure the length and width of the room and the height of the walls.
- Enter how many doors and windows the room has. Their area is subtracted from the walls.
- Choose the number of coats. Two coats is typical, especially when changing color.
- Enter the coverage from your paint can. If you do not know it, the defaults (350 sq ft per gallon, 10 m² per litre) are reasonable for smooth, previously painted walls.
The formula
Wall area = 2 × (length + width) × wall height − doors − windows Paint needed = (wall area + ceiling area) × coats ÷ coverageWorked example
A 12 × 12 ft bedroom with 8 ft walls, one door and two windows:
- Walls: 2 × (12 + 12) × 8 = 384 sq ft.
- Minus one door (21 sq ft) and two windows (30 sq ft) = 333 sq ft.
- Two coats: 666 sq ft ÷ 350 = 1.9 gallons, so you would buy 2 gallons.
What changes how much paint you use
- Surface texture. Rough plaster, textured walls and bare masonry absorb more paint than smooth drywall.
- New drywall or a big color change. A coat of primer first usually means fewer top coats.
- Application method. Spraying typically uses more paint than rolling because of overspray.
Frequently asked questions
How much does a gallon of paint cover?
Most interior wall paints list roughly 250–400 square feet per gallon for one coat on a smooth surface. Porous or textured surfaces get less. The exact figure is printed on the can.
Should I subtract doors and windows?
Yes for an accurate estimate. This calculator subtracts a standard door and window size for each one you enter. If your openings are much larger, such as patio doors, reduce the wall size or add them as extra doors.
How many coats of paint do I need?
Two coats is the usual recommendation for even color. One coat can be enough when repainting the same color, while very dark-to-light changes may need primer plus two coats.
Is ceiling paint calculated separately?
Tick “Include the ceiling” to add the floor area of the room to the total. If you are using a different product for the ceiling, run the calculator twice: once for the walls and once for the ceiling only.