Guide
Buying paint is a balancing act: too little means a mid-project store run and the risk of a slightly different mixing lot, while too much leaves you with cans crowding the garage. The good news is that paint quantity is pure arithmetic. Once you know the coverage rule and a couple of adjustments, you can nail it every time.
One gallon of interior paint covers roughly 350 to 400 square feet in a single coat on a smooth, primed surface. That's the anchor for every paint calculation. Rough, porous, or unprimed surfaces soak up more and cover less, so lean toward the lower end (350) when in doubt.
For each wall, multiply width by height, then add the walls together. A 12 ft × 12 ft room with 8 ft ceilings has four walls totaling about 384 sq ft of surface (perimeter of 48 ft × 8 ft height).
You don't paint the glass or the door slab, so subtract them. Use handy averages: about 20 sq ft per door and 15 sq ft per window. One door and two windows would trim roughly 50 sq ft off the example above, leaving about 334 sq ft to paint.
Divide your paintable area by 350–400 to get gallons per coat, then multiply by the number of coats. Most color changes and fresh walls need two coats. Our example: 334 sq ft ÷ 350 ≈ 1 gallon per coat, so about 2 gallons for two coats. Always round up to whole cans.
| Step | Example |
|---|---|
| Wall area | 384 sq ft |
| Minus 1 door + 2 windows | −50 sq ft → 334 sq ft |
| Gallons per coat (÷350) | ≈ 1 gallon |
| Two coats | 2 gallons |
Bare drywall, patched spots, or a dramatic color change usually call for primer, which follows the same coverage math. Ceilings are calculated separately as length × width. Trim and doors use small quantities but shouldn't be skipped in your budget.
To skip the pencil work, the paint calculator lets you enter room dimensions, doors, windows, and coats, then returns the exact number of gallons — coverage math and rounding handled for you.