Guides & answers
All 10 guides →How to figure out how much paint to buy
Short answer: multiply the paintable wall area by the number of coats, divide by the coverage on the can (about 350 sq ft per gallon), then buy the cheapest cans that cover it. That last step is what CoatMath adds.
The formula
Worked example: a 12 × 12 ft room with 8 ft walls
| Step | Result |
|---|---|
| Gross wall area: 2 × (12 + 12) × 8 | 384 sq ft |
| Minus 1 door (20) and 2 windows (15 each) | 334 sq ft |
| Two coats at 350 sq ft per gallon | 334 × 2 ÷ 350 = 1.91 gallons |
| Buy this | 2 gallons, about $90 |
Coverage figures follow PaintCare’s Buy Right guide: most cans are rated 300 to 400 sq ft per gallon, and 350 is a fair rough estimate. Prices are typical 2026 US figures; edit them to your store’s.
Why CoatMath
- One answer: “2 gallons + 1 quart”, not just “2.1 gallons”.
- Paint, primer and ceiling paint as separate lines, because they are different cans.
- Both options shown: the cheapest mix and the round-up-to-whole-gallons price.
- Share, embed and print a shopping list. No ads.
Frequently asked questions
How much paint do I need?
Wall area (perimeter × height, minus doors and windows) × coats ÷ coverage per gallon. A 12 × 12 ft room with 8 ft walls, one door and two windows has 334 sq ft of wall, so two coats at 350 sq ft per gallon is 1.91 gallons: buy 2 gallons.
How much does a gallon of paint cover?
Most cans say 300 to 400 square feet per gallon, and PaintCare suggests using 350 for a rough estimate. Rough textures can need up to 50 percent more paint. A quart covers a quarter of that.
How many gallons of paint for a 12x12 room?
About 2 gallons for the walls with two coats (1.91 gallons exact), and 1 more gallon if you also paint the ceiling.
Is a 5-gallon bucket cheaper than gallons?
Often. At typical 2026 prices ($45 a gallon, $200 a bucket) a bucket costs the same as four gallons plus a quart, so once you need more than 4 gallons the bucket gives you 5 for the price of 4 and a quart. CoatMath compares them for you.
Do I need primer?
Not always. New drywall, stains, glossy surfaces and big color changes usually do. Add a primer line in the calculator to see the cans and cost.
Are the prices accurate?
They are typical 2026 US mid-grade interior prices, labeled as such, and every one is editable. Type your store’s price for an exact total.
Does CoatMath store my measurements?
No. Everything stays in your browser. Your numbers appear in the page link only so you can share the result.
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