Paint quantity calculator
Enter your room and get the gallons. The maths is below the form so you can check it, and the reference table works with JavaScript off.
Your room
Result: —
The formula
Gross wall area = perimeter × height. Subtract 20 sq ft per door and 15 sq ft per window. Multiply by coats, divide by the spread rate, round up to whole gallons.
Worked example
A 12 ft × 8 ft bedroom with 8 ft walls has a 40 ft perimeter, so 40 × 8 = 320 sq ft gross. One door and two windows remove 20 + 30 = 50, leaving 270 sq ft. Two coats is 540 sq ft. At 350 sq ft per gallon that is 1.54 gallons — buy 2.
Assumptions
The 350 sq ft per gallon default is the general industry figure, not a product specification. Use the spread rate on your product's technical data sheet for a real number. Textured or unprimed surfaces cover less, sometimes by a third or more.
Reference table (two coats, 350 sq ft/gal)
| Room | Perimeter | Wall area | Gallons |
|---|---|---|---|
| 10 × 10 | 40 ft | 270 sq ft | 2 |
| 12 × 12 | 48 ft | 334 sq ft | 2 |
| 12 × 16 | 56 ft | 398 sq ft | 3 |
| 15 × 20 | 70 ft | 510 sq ft | 3 |
| 20 × 24 | 88 ft | 654 sq ft | 4 |