Buying paint is a guessing game when you only know the room size. This calculator turns length, width and wall height into the actual paintable area, takes out doors and windows, multiplies by the number of coats and divides by your paint's coverage — so you know whether one gallon, two or three will finish the job without an extra trip.
How to use the paint calculator
- Measure the room along the floor: length and width. Then measure wall height from floor to ceiling.
- Decide about the ceiling. Choose “walls and ceiling” only if the ceiling gets the same paint.
- Count doors and windows. The calculator subtracts 20 sq ft per door and 15 sq ft per window.
- Set the coats. Two is the safe default; three for a dramatic color change.
- Check the coverage on your can and enter it. If you have not chosen a paint yet, 350 sq ft per gallon is a reasonable middle value.
“Gallons to buy” rounds up to whole gallons. If the exact figure is just over a whole number, the quarts figure tells you whether a gallon plus a quart or two would do.
Paint calculation formula
Paintable area = wall area + ceiling − (doors × 20) − (windows × 15)
Gallons = paintable area × coats ÷ coverage per gallon
- L, W — room length and width
- H — wall height
- Ceiling — L × W, included only if you paint it
- Coverage — square feet one gallon covers in one coat, from the label
The 20 and 15 sq ft allowances match a standard 36 × 80 in door and a typical 3 × 5 ft window. Large picture windows or sliding doors are bigger — measure those and adjust.
Worked example: a 12 × 10 ft bedroom
- Wall area: 2 × (12 + 10) × 8 = 352 sq ft
- Subtract one door and two windows: 352 − 20 − 30 = 302 sq ft
- Two coats: 302 × 2 = 604 sq ft of coverage needed
- At 350 sq ft per gallon: 604 ÷ 350 = 1.73 gallons
Buy 2 gallons. The leftover quarter gallon is useful for touch-ups — keep it sealed and labeled with the room.
Paint needed for common room sizes
Walls only, two coats, 350 sq ft per gallon, nothing subtracted for doors or windows.
| Room | Wall height | Wall area | Gallons (exact) | Gallons to buy |
|---|---|---|---|---|
| 10 × 10 ft | 8 ft | 320 sq ft | 1.83 | 2 |
| 10 × 12 ft | 8 ft | 352 sq ft | 2.01 | 3 |
| 12 × 12 ft | 8 ft | 384 sq ft | 2.19 | 3 |
| 12 × 14 ft | 8 ft | 416 sq ft | 2.38 | 3 |
| 14 × 16 ft | 8 ft | 480 sq ft | 2.74 | 3 |
| 14 × 16 ft | 9 ft | 540 sq ft | 3.09 | 4 |
| 16 × 20 ft | 8 ft | 576 sq ft | 3.29 | 4 |
| 16 × 20 ft | 9 ft | 648 sq ft | 3.70 | 4 |
Rows like the 10 × 12 ft room land just over 2 gallons; subtracting a door and a window brings them back under, which is why the inputs for openings are worth filling in.
What changes how much paint you use
- Surface texture. Fresh drywall, textured walls, brick and stucco soak up paint. Prime new drywall first; otherwise expect noticeably lower coverage than the label.
- Color change. Going from dark to light usually needs a tinted primer plus two coats.
- Application method. A sprayer can use considerably more paint than a roller because of overspray.
- Trim and doors. Baseboards, casings and doors are usually painted in a different sheen; estimate them separately.
Covering walls with new panels first? The Drywall Calculator counts sheets. Adding wall trim? Try the Board and Batten Calculator.
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. Two coats at 350 sq ft per gallon is 384 × 2 ÷ 350 ≈ 2.2 gallons, so buy 3 gallons. Subtract one door and two windows and it drops to about 1.9 gallons, so 2 gallons is enough.
How many square feet does a gallon of paint cover?
Most interior wall paints list 250–400 sq ft per gallon for one coat on a smooth, primed surface. Rough or porous surfaces like new drywall, brick or stucco take more. Use the number printed on your can.
Do I need two coats of paint?
Usually yes. Two coats give even color and hide roller marks, especially when changing colors. One coat can be enough when repainting the same color over a sound surface, or with paint-and-primer products on light colors.
Should I subtract doors and windows?
Yes for an accurate estimate. A standard door is about 20 sq ft and an average window about 15 sq ft. For a small room the difference is often under a quart, so many painters skip it and treat the extra as touch-up paint.
How much paint for a ceiling?
A ceiling is just length × width. A 12 × 10 ft ceiling is 120 sq ft, so one coat at 350 sq ft per gallon needs about a third of a gallon. Ceiling paint is usually bought separately in flat white.