Paint Calculator
This paint calculator works out how many gallons you need to paint a room. Enter the room size and ceiling height, take off the doors and windows, and pick your coats, and you'll get the gallons to buy along with the paintable square footage. It uses the standard 350 square feet per gallon, which you can change to match your paint can.
- Gallons for a room
- Doors & windows removed
- Any coverage & coats
- Optional ceiling
- Live as you type
Last updated June 17, 2026 1 gallon ≈ 350 sq ft per coat Reviewed by the Calcowa team
Enter the room size above 0.
Doors count as ~21 sq ft and windows ~15 sq ft. Estimate for planning; buy a little extra for touch-ups.
312 ft² × 2 coats ÷ 350 = 1.78 → 2 gallons
How much paint do I need?
Paint is sold by coverage, so the job comes down to area. Find the wall area by multiplying the room's perimeter by the ceiling height, subtract the doors and windows you won't paint, then divide by the coverage on the can, usually about 350 square feet per gallon. Multiply by the number of coats, and round up to whole gallons, since you can't buy a fraction of a can. A 12 by 12 room with 8-foot walls is 384 square feet, and after openings and two coats it's near 2 gallons. The calculator runs all of that the moment you type, so you'll know what to buy before you're at the store.
Working out a room, step by step
Here's how the calculator handles a 12 by 12 room with 8-foot ceilings, one door and two windows, two coats. It's the same path you'd take by hand, just faster:
- 1
Find the wall areaPerimeter is 2 × (12 + 12) = 48 ft, times the 8-foot height, so 384 square feet of wall.
- 2
Subtract the openingsOne door (21) and two windows (30) remove 51 square feet, leaving 333 to paint.
- 3
Divide by coverage333 ÷ 350 is about 0.95 gallons for a single coat.
- 4
Multiply by coats and round upTwo coats is 1.9 gallons, so you'll buy 2 gallons.
How coats and coverage change the total
Two numbers drive the amount: how many coats you apply and how far the paint spreads. Two coats is standard for an even, lasting finish, and it's basically double the paint of a single coat. Coverage varies by paint and surface, since a smooth primed wall takes the full 350 square feet a gallon, while bare drywall or a rough texture drinks more and you'll get less out of the can. A bold color change over a dark wall often needs an extra coat too, so don't skimp there. Set both above to match your job, and you'll see the gallons shift. To measure the room first, the square footage calculator gives you the floor and wall areas.
Frequently asked questions
How much paint for a 12x12 room?
A 12 by 12 room with 8-foot walls is about 384 square feet, so roughly 1 gallon for one coat or 2 gallons for two coats, a touch less once you remove the door and windows. That's a common size, which is why 2 gallons covers most bedrooms.
Measure the wall area, subtract doors and windows, then divide by the coverage of your paint, usually about 350 square feet per gallon, and multiply by the number of coats. A 12 by 12 room with 8-foot walls has 384 square feet, and after two coats it needs roughly 2 gallons. This paint calculator does that for you, so you'll buy close to the right amount.
Coverage is how far a gallon stretches, printed on the can as square feet per gallon. Most interior wall paints cover about 350 square feet in one coat, though primers and rough surfaces cover less. Divide your paintable area by the coverage to get gallons per coat, then multiply by the coats. The calculator above uses 350 by default, and you can change it to match your paint.
Two coats is the norm for a durable, even finish, and it's what most pros quote. One coat works for a small color refresh over a similar shade, while a big color change, a patchy wall, or bare drywall wants two coats over primer. Set the number of coats above and the gallons update, since each extra coat adds the same amount of paint again.
Often, for one coat of a small to mid-size room. A gallon covers about 350 square feet, which is roughly the walls of a 10 by 10 room. For two coats, or a larger space, you'll want two gallons. Buying by the gallon is cheaper per square foot than quarts, so round up to the next gallon unless you only need a touch.
Yes, for a tighter estimate. A standard door is about 21 square feet and an average window about 15, so a couple of each removes 70-plus square feet from the total. On a small room that's nearly a quarter of a wall, enough to change whether you need a second gallon. The calculator subtracts them once you enter the counts.
A ceiling takes its area, length times width, divided by the same 350-square-foot coverage. A 12 by 12 ceiling is 144 square feet, so under half a gallon for one coat, though ceilings often want two. Tick the ceiling option above to fold it into the total, since ceiling and wall paint are usually bought separately.
Related calculators
More tools for rooms and materials.
Area of a room or house.
Tile calculatorTiles and boxes for a floor.
All construction calculatorsConcrete, gravel, paint, tile.
Painting a room?
Size the job above, or browse the full construction hub.