About this calculator
This paint calculator measures the wall area of a rectangular room, subtracts standard-size doors and windows, multiplies by the number of coats and divides by your paint’s coverage to tell you exactly how many gallons — and quarts — to buy. It can add the ceiling as a separate figure, since ceilings usually get a different paint.
How to use it
- Enter the room’s length, width and wall height.
- Enter the number of doors and windows. Very large openings (patio doors, picture windows) can be counted as two.
- Choose the number of coats. Two coats is standard; one may do when repainting the same color.
- Check the coverage printed on the can (usually 250–400 sq ft per gallon) and enter it.
The formula
Wall area = 2 × (length + width) × height
Paintable area = wall area − doors × 21 − windows × 15
Gallons = paintable area × coats ÷ coverage per gallon
If the result is a quarter-gallon or less over a whole number, the calculator suggests adding a quart instead of another gallon. Beyond that, a second gallon usually costs about the same as two or three quarts.
Worked example
These are the calculator’s starting values and the results they produce:
| Room length | 14 ft |
|---|---|
| Room width | 12 ft |
| Wall height | 8 ft |
| Doors | 1 |
| Windows | 2 |
| Coats | 2 |
| Coverage per gallon | 350 sq ft |
| Paint the ceiling too? | No, walls only |
Tips before you buy
- Use primer (and count it as a coat) on new drywall, bare wood, stains or when changing from a dark to a light color.
- Textured walls, rough plaster and flat paint on porous surfaces can use 10–20% more paint — lower the coverage figure.
- Buy all your paint at once and box it (mix the cans together) so the color is identical throughout.
- Keep a labeled half-pint of leftover paint for touch-ups.
Frequently asked questions
How much paint do I need for a 12×12 room?
A 12 × 12 ft room with 8 ft walls has 384 sq ft of wall. After one door and two windows (51 sq ft) that is 333 sq ft, so two coats at 350 sq ft per gallon need about 1.9 gallons — buy 2 gallons.
How many square feet does a gallon of paint cover?
Most interior paints cover 250–400 sq ft per gallon per coat on smooth, primed walls. Check the label — coverage drops on textured or porous surfaces.
How much paint do I need for the ceiling?
A ceiling’s area is simply the room’s length × width. A 12 × 14 ft ceiling is 168 sq ft, so two coats at 350 sq ft per gallon need just under 1 gallon.
Do I need two coats of paint?
Usually yes. Two coats give even color and sheen, especially over a different color. One coat can be enough when refreshing walls in the same color with a good-quality paint.