How to use the paint calculator
- Measure the room's length and width along the floor, and the wall height from floor to ceiling.
- Count the doors and windows. The calculator subtracts 21 sq ft per door (a standard 3 × 7 ft door) and 15 sq ft per window.
- Choose the number of coats. Two coats is standard; one may do for a fresh coat in the same color, three for a big color change without primer.
- Check the coverage printed on your paint can and enter it if it's different from 350 sq ft per gallon.
How much paint you need: the formula
wall area = 2 × (length + width) × height − doors − windows
gallons = wall area × coats ÷ coverage per gallon
Example: a 12 × 12 ft bedroom with 8 ft walls has 384 sq ft of wall. Minus one door (21 sq ft) and two windows (30 sq ft), that's 333 sq ft. Two coats at 350 sq ft per gallon is 1.9 gallons, so buy 2 gallons.
Paint needed for common room sizes
Two coats, one door, two windows, 350 sq ft per gallon, walls only:
| Room | Paintable wall (sq ft) | Gallons (2 coats) |
|---|---|---|
| 10 × 10 ft, 8 ft ceiling | 269 | 1.5 |
| 10 × 12 ft, 8 ft ceiling | 301 | 1.7 |
| 12 × 12 ft, 8 ft ceiling | 333 | 1.9 |
| 12 × 14 ft, 8 ft ceiling | 365 | 2.1 |
| 14 × 16 ft, 8 ft ceiling | 429 | 2.5 |
| 12 × 12 ft, 9 ft ceiling | 381 | 2.2 |
| 16 × 20 ft, 9 ft ceiling | 597 | 3.4 |
| 20 × 20 ft, 10 ft ceiling | 749 | 4.3 |
Tips for buying paint
- Gallons vs quarts: a quart is a quarter gallon, but two or three quarts often cost as much as a gallon. If you need more than half a gallon extra, buy the gallon.
- 5-gallon buckets are usually 10–20% cheaper per gallon than single cans when you need 4 or more gallons of one color.
- Textured or porous walls (new drywall, stucco, brick) soak up more paint. Lower the coverage to 250–300 sq ft per gallon, and prime new drywall first.
- Keep a little left over for touch-ups, and write the room and color code on the lid.
Frequently asked questions
How much paint do I need for a 12x12 room?
About 2 gallons for two coats on the walls of a 12 × 12 ft room with 8 ft ceilings, one door and two windows. Add about 1 gallon if you also paint the 144 sq ft ceiling with two coats.
How many square feet does a gallon of paint cover?
Most interior paints cover 350–400 sq ft per gallon on smooth, primed walls. Rough, porous or unprimed surfaces can drop that to 250 sq ft or less.
Do I need two coats of paint?
Usually yes. Two coats give even color and a durable finish. One coat can be enough when repainting the same color with a quality paint.
How much paint for a ceiling?
Multiply the room length by width to get the ceiling area, then divide by the coverage per gallon for each coat. A 12 × 12 ceiling is 144 sq ft, so about 0.8 gallons for two coats.