Add up the walls (perimeter × height), take off 21 sq ft for each door and 15 for each window, multiply by the number of coats and divide by 350 sq ft per gallon, Sherwin-Williams’ planning figure. A 12 × 12 ft bedroom with 8 ft walls, one door and two windows has 333 sq ft of wall: two coats take 1.9 gallons, so buy 2 gallons.
How to measure a room for paint
- Length and width: measure the room along the floor, wall to wall. For a room that isn’t a rectangle, add up the width of every wall and use One wall with that total.
- Wall height: floor to ceiling. Measure it rather than assume 8 ft: 9 ft walls take about an eighth more paint.
- Doors and windows: count them. The calculator takes off Sherwin-Williams’ standard 21 sq ft per door and 15 sq ft per window, so you don’t need to measure each one. Leave out a door or window you will paint in the wall color.
- Ceiling: length × width. The calculator lists it on its own, in case you use a different paint up there.
The math, with your numbers
These lines follow whatever you typed into the calculator above.
The calculator buys whole gallons, adding a quart when the leftover is a quarter gallon or less. A quart is a quarter of a gallon and covers about 100 sq ft, says Sherwin-Williams.
How much does a gallon of paint cover?
Sherwin-Williams says a gallon typically covers about 350 to 400 sq ft, and its painter training course plans every coat at 350. The coverage printed on a can is figured at the recommended film thickness, so real walls usually take a little more. Sherwin-Williams calls the label figure theoretical, because the texture of the surface and the paint left on the roller aren’t counted.
| Product | Covers per gallon | Source |
|---|---|---|
| Planning figure, per coat | 350 sq ft | Sherwin-Williams training course |
| Typical gallon of paint | 350 to 400 sq ft | Sherwin-Williams |
| Regal Select interior | 400 to 450 sq ft | Benjamin Moore data sheet |
| ben interior | 400 to 450 sq ft | Benjamin Moore sell sheet |
| A quart of paint | about 100 sq ft | Sherwin-Williams |
| A gallon of primer | 200 to 300 sq ft | Sherwin-Williams |
The calculator starts at 350, the cautious end. If your can says 400, type it in; a bare, rough or textured surface will use more than the label says, so stay at 350 or below for those.
How much paint for a room
Walls 8 ft high, one door and two windows, two coats at 350 sq ft per gallon:
| Room | Wall to paint | Wall paint | Buy | Ceiling, 2 coats |
|---|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1.54 gal | 2 gallons | 1 gallon |
| 10 × 12 ft | 301 sq ft | 1.72 gal | 2 gallons | 1 gallon |
| 12 × 12 ft | 333 sq ft | 1.90 gal | 2 gallons | 1 gallon |
| 12 × 14 ft | 365 sq ft | 2.09 gal | 2 gallons + 1 quart | 1 gallon |
| 14 × 16 ft | 429 sq ft | 2.45 gal | 3 gallons | 2 gallons |
| 16 × 20 ft | 525 sq ft | 3.00 gal | 3 gallons | 2 gallons |
Our arithmetic with Sherwin-Williams’ method. With 9 ft walls, the 12 × 12 ft room has 381 sq ft to paint and takes 2.18 gallons.
How many coats of paint?
Plan on two. Benjamin Moore’s data sheet for Regal Select says bare surfaces need two coats and previously painted walls can be finished with one or two, and it recommends two to get full color and the best film performance. Sherwin-Williams’ estimating method counts one coat at a time on smooth, nonporous walls, so multiply by the coats you plan.
The same data sheet notes that some deep colors need a tinted primer underneath to reach the right shade. If a color change isn’t covering after two coats, the calculator also does a third.
How much primer do I need?
Primer goes on as one coat, and a gallon covers less than paint: about 200 to 300 sq ft, according to Sherwin-Williams. The calculator uses 250. Sherwin-Williams’ training notes it is “especially vital to prime when the surface is bare,” such as new drywall or bare wood. Many paints are sold as paint and primer in one, and Benjamin Moore describes Regal Select as self-priming on most surfaces; check the can before buying a separate primer for walls that are already painted.
For the 12 × 12 ft room above, one coat of primer on 333 sq ft is 1.33 gallons at 250 sq ft per gallon: a gallon and two quarts, or simply 2 gallons.
Ceilings, doors and accent walls
- Ceiling: length × width. A 12 × 12 ft ceiling is 144 sq ft: 0.82 gallons for two coats, so one gallon.
- A door: Sherwin-Williams counts a door as 21 sq ft, the face of a 3 × 7 ft door. Both sides with two coats is 84 sq ft, which a quart covers.
- An accent wall: a 10 ft wall 8 ft high is 80 sq ft; two coats take 0.46 gallons, so two quarts or one gallon. Pick One wall in the calculator.
- Exterior walls: use the coverage on the can, and expect rough surfaces such as stucco and masonry to take more than smooth drywall, since the label figure leaves out texture.
Write the room, color name and sheen on the lid before you put leftovers away. A labeled can is what you will want the next time a wall gets scuffed.
Questions people ask
How much paint do I need for a 12 × 12 room?
About 2 gallons for two coats on 8 ft walls with one door and two windows: 333 sq ft of wall × 2 coats ÷ 350 = 1.9 gallons. Add a gallon if you are painting the ceiling too.
How much does a gallon of paint cover?
About 350 to 400 sq ft per coat, says Sherwin-Williams; some premium paints list 400 to 450. Plan with 350 unless your can says otherwise.
How much paint do I need for a bedroom?
Two gallons covers two coats in a bedroom up to about 12 × 12 ft with 8 ft walls, one door and two windows. A 12 × 14 ft room needs a quart more, and 14 × 16 ft three gallons.
How many coats of paint do I need?
Two. Benjamin Moore requires two on bare surfaces and recommends two for full color; on previously painted walls its data sheet allows one or two.
How much primer do I need?
Divide the area by 250 for one coat: a gallon of primer covers about 200 to 300 sq ft. A 12 × 12 ft room’s walls take about 1⅓ gallons.
How much paint do I need for a door?
A quart. Both sides of a 3 × 7 ft door with two coats is 84 sq ft, and a quart covers about 100.
Should I subtract windows and doors?
Yes, if they won’t be painted in the wall color. Sherwin-Williams takes off 21 sq ft per door and 15 sq ft per window.
Sources
- Sherwin-Williams, Painter Skills Program, Part 1 (2023): how much paint, door and window allowances, the worked example, priming bare surfaces
- Sherwin-Williams, Paint calculator FAQs (gallon, quart and primer coverage) and Painting FAQs (label coverage is theoretical)
- Benjamin Moore, Regal Select Pearl (550) technical data sheet (coverage, coats, self-priming)
- Benjamin Moore, ben interior sell sheet (coverage)
Checked October 12, 2026. Room totals are our arithmetic. Metric values are converted from square feet and US gallons (3.785 L).