Paint Calculator
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The paint calculator shows how much paint you need and how many gallons to buy: (wall area − doors − windows) × coats ÷ coverage per gallon. A 12' × 14' room with 8' walls, one door and two windows has 366 ft² to paint. Two coats at 350 ft² per gallon take 2.09 gal, or 2 gal + 1 qt. With the default 10% extra, buy 2 gal + 2 qt.
This paint calculator online has two modes. Room is the default: you type the size of the room and count the openings, and the wall area is worked out for you. Wall area takes a number you already have, for a facade, a fence or a single wall.
How much paint do I need for a room?
How many gallons of paint do I need for one room? Enter Length, Width and Wall height, then the number of Doors and Windows. The calculator multiplies the perimeter by the wall height and takes the openings out:
(2 × (length + width) × wall height − doors × 20 − windows × 15) × coats ÷ coverage = gallons
For the 12' × 14' room with 8' walls, that's 2 × (12 + 14) × 8 = 416 ft² of wall. One door takes off 20 ft² and two windows 30 ft², which leaves 366 ft². At two coats and 350 ft² per gallon: 366 × 2 ÷ 350 = 2.09 gal.
The opening sizes are shown under the fields, and you can change them. 20 ft² is a 36" × 80" door. 15 ft² is a 3' × 5' window — change it if yours are bigger.
Room is where the tool opens, so it works as a room paint calculator straight away, and as an interior paint calculator for any space with four walls. The big number on the display is what you buy. Paint comes in gallons and quarts, so a paint quantity calculator has to end in cans: 2.09 gal is rounded up to the next quart and shown as 2 gal + 1 qt. That's the part a how much paint do I need calculator usually leaves to you.
We add 10% by default, because running out in the middle of the second coat means another trip to the store. With 10% the same room is 2.30 gal, or 2 gal + 2 qt, and the exact line reads 2.30 gal · incl. 10% extra. Set Extra % anywhere from 0 to 50; 0 means no extra.
On metric the result comes in 1 L, 2.5 L and 5 L cans. The same room is 7.92 L, bought as 5 L + 2.5 L + 1 L; with 10% it's 8.71 L, or 5 L + 2.5 L + 2 × 1 L.
Want the cost? Enter your price per gallon and per quart. The result adds a Paint cost line: gallons × your price per gallon + quarts × your price per quart. Leave the quart price empty and a quart counts as a quarter of the gallon price.
Paint by square footage
Already measured the walls in square feet? Switch to Wall area and type the total. That's the whole paint calculator: square feet in, gallons out. At 350 ft² per gallon and two coats, one gallon covers 175 ft² of wall.
Example: 400 ft² of wall, two coats, 10% extra comes to 2.51 gal. Rounded up by the quart that would be 2 gal + 3 qt, and three quarts usually cost more than a gallon, so the display shows 3 gal. The exact 2.51 gal stays on the line below.
For a gable, a stair wall or a room with odd corners, work out the wall area in the square footage calculator, then enter it in Wall area. The number doesn't carry over by itself; you type it in.
Paint coverage per gallon
Coverage is how many square feet one gallon covers in one coat. Behr and Benjamin Moore data sheets list 250 to 450 ft² per gallon; we start at 350 — use the number on your can. Where your paint falls in that range depends on how you apply it and how porous the surface is.
The Coverage field sits under the inputs with the default already filled in. Type the figure from the label, and the tool becomes a paint coverage calculator for that exact paint. Change 350 to 400 and every gallon covers more wall; drop it to 250 and you'll need more cans.
Turned around, it's a gallon of paint calculator: one gallon at 350 ft² covers 175 ft² of wall at two coats. On metric the default is 8.6 m²/L, the same 350 ft²/gal converted.
Walls and ceiling
The wall paint calculator is the Room mode as it opens: four walls, minus doors and windows. Painting one accent wall? Switch to Wall area and enter that wall's square feet. Used one wall at a time like this, it's a paint calculator for walls.
Tick Include ceiling and the ceiling, length × width, is added to the walls with the same paint, coats and coverage. In the 12' × 14' room that's 366 + 168 = 534 ft², shown as 534 ft² × 2 coats. Two coats come to 3.05 gal, or 3 gal + 1 qt; with 10% extra it's 3.36 gal, or 3 gal + 2 qt.
If you use a separate ceiling paint, run the ceiling on its own in Wall area. As a ceiling paint calculator: enter 168 ft² for the 12' × 14' ceiling, two coats, and you get 0.96 gal, or 1 gal. With 10% extra, 1 gal + 1 qt. Add the walls as a second calculation without Include ceiling.
Exterior paint
The exterior paint calculator is the Wall area mode. Measure the sides of the house you're painting, subtract the doors and windows, and enter the total. That's how it works as a house paint calculator: you bring the area, and the tool turns it into gallons for your coats and coverage.
Same for an outside paint calculator job that isn't a house, like a fence or a garage wall: use Wall area.
Coverage outside is lower than on smooth drywall. Rough siding, stucco and brick soak up more paint, so take the low end of the range on the can and type it into Coverage.
Primer
Tick Primer and a second line appears under the paint: one coat, with its own coverage. The primer calculator starts at 300 ft² per gallon. KILZ lists 300 to 400 ft² per gallon of primer; we take the low end because new drywall soaks it up. For new drywall, Behr's data sheet calls for 1 coat of primer and 2 coats of paint.
On the same 366 ft² of walls, one coat at 300 ft² per gallon is 1.22 gal, or 1 gal + 1 qt. With 10% extra, 1 gal + 2 qt. Used as a drywall primer calculator, that's the line to take to the store. Primer has no price field, so it isn't part of Paint cost.
What's next
Press Add to project under the result. The paint goes to My project as a line, for example Room 12' × 14' · 366 ft² × 2 coats · 2 gal + 2 qt, plus the cost if you entered a price. Primer goes in as its own line, Primer · Room 12' × 14' · 1 gal + 2 qt, without a price. Tap a name to rename the line.
Lines from different calculators sit in one list, with a total per material at the bottom, and you can print it. The numbers you enter stay in your browser; no account needed. Same job needs a floor area or a slab? Our other construction calculators add to the same list.
FAQ
How much paint do I need for a 12x12 room?
About 2 gallons for two coats. With 8' walls, one door and two windows: 384 − 20 − 30 = 334 ft², and 334 × 2 ÷ 350 = 1.91 gal, so 2 gal. With 10% extra it's 2.10 gal, or 2 gal + 1 qt.
How many square feet does a gallon of paint cover?
Behr and Benjamin Moore data sheets list 250 to 450 ft² per gallon for one coat; we start at 350 — use the number on your can. At two coats, a gallon at 350 covers 175 ft² of wall.
How many coats of paint do I need?
Two. Benjamin Moore's data sheet recommends two coats for full color, and Behr's calls for two coats of paint over new drywall. Coats is 2 by default; set it from 1 to 5.
How much paint do I need for a ceiling?
A 12' × 14' ceiling is 168 ft². Two coats at 350 ft² per gallon take 0.96 gal, so 1 gal, or 1 gal + 1 qt with 10% extra. Same paint as the walls? Tick Include ceiling instead.
How much primer do I need?
One coat. At 300 ft² per gallon, the low end of the 300 to 400 KILZ lists, the 366 ft² of walls in a 12' × 14' room take 1.22 gal: 1 gal + 1 qt, or 1 gal + 2 qt with 10% extra.
Results are estimates for ordering material. Check the quantity with your supplier before you order.