How to calculate how much paint you need
Work out the wall area, subtract doors and windows, multiply by the number of coats and divide by the coverage of a gallon. One gallon covers about 350 to 400 square feet.
The formula
Gallons = (wall area − doors − windows) × coats ÷ 350
Worked example
A 12 by 15 foot room with 8 foot ceilings has 432 square feet of wall. Subtracting one door (20 square feet) and two windows (30 square feet in total) leaves 382 square feet. Two coats need 764 square feet of coverage, so buy 3 gallons.
How to use this calculator
Enter the room length, room width and wall height, the number of doors and windows, and the number of coats. Choose whether to paint the ceiling and enter the coverage per gallon from your paint’s label. The paint calculator subtracts the openings, multiplies by the coats and divides by that coverage, then shows the wall area, the wall paint, the ceiling paint and the primer for one coat. Round up when you buy, because a partly used can is better than stopping in the middle of a wall.
What the result means
The result is the paint needed, with wall paint and ceiling paint shown separately. It uses the coverage per gallon you enter. A figure of 350 square feet per gallon is the low end of the usual range, so it leans toward having enough. The label on your paint gives the coverage for that product. If it is closer to 400 square feet, you may need less.
Ceiling paint appears only if you choose to paint the ceiling. Trim and doors are separate jobs, often in a different paint, so work those out on their own.
More examples
The same room with different coats and coverage
This table uses the 382 square feet of wall from the worked example.
| Coats | Area to cover | Gallons at 350 sq ft | Gallons at 400 sq ft |
|---|---|---|---|
| 1 | 382 sq ft | 1.09, buy 2 | 0.96, buy 1 |
| 2 | 764 sq ft | 2.18, buy 3 | 1.91, buy 2 |
| 3 | 1,146 sq ft | 3.27, buy 4 | 2.87, buy 3 |
Coverage makes a full gallon of difference here, which is why the figure on the can matters.
A small bedroom
A 10 by 10 foot room with 8 foot ceilings has 40 feet of wall, so 40 × 8 = 320 square feet. Subtract one door (20 square feet) and one window (15 square feet) to get 285 square feet. Two coats need 570 square feet of coverage. 570 ÷ 350 = 1.63, so buy 2 gallons.
The ceiling
The ceiling of the 12 by 15 foot room is 180 square feet. Two coats cover 360 square feet, and 360 ÷ 350 = 1.03 gallons. One gallon will not quite stretch at that coverage, so buy a gallon and a quart, or check whether your paint covers closer to 400 square feet.
What changes the result
- Coats. Each coat uses the full wall area again. Two coats take twice the paint of one.
- Ceiling height. Wall area rises in step with height. The same room with 10 foot ceilings has 540 square feet of wall instead of 432.
- The surface. Rough, textured or unpainted surfaces soak up more paint than smooth, previously painted walls.
- Openings. Large windows, patio doors and built-in cabinets reduce the area.
Common mistakes
- Using the floor area of the room instead of the wall area.
- Forgetting to multiply by the number of coats.
- Leaving the ceiling, closets or trim out of the plan and running short.
- Buying cans at different times. Color can vary slightly between batches, so buy the full amount together and mix the cans before you start.
- Rounding down to save a can.
Related calculators
The square footage calculator helps with walls and ceilings that are not plain rectangles. If the room is getting new floors as well, see the flooring calculator, and use the renovation cost calculator to budget the whole project.
Frequently asked questions
How do I find wall area?
Add the lengths of all walls and multiply by the ceiling height.
Do I need primer?
Yes on new drywall, on stains, and when making a large color change.
How much paint for a ceiling?
Length × width of the room, divided by 350 per coat.
How much paint do I need for a 10 by 10 room?
With 8 foot ceilings, one door and one window, the walls come to about 285 square feet. Two coats take about 1.6 gallons, so buy 2 gallons.
Should I subtract doors and windows?
Yes, if you want the closest estimate. If you leave small openings in, you simply end up with a little spare paint for touch-ups.
How do I measure a wall with a sloped ceiling?
Split it into a rectangle and a triangle. The triangle’s area is half its base times its height. A triangle 12 feet wide and 4 feet high is 24 square feet.
How many quarts are in a gallon?
Four. A quart covers about a quarter of what a gallon does, which makes it useful for trim or a small top-up.