How to use the Paint Calculator
- Choose feet and gallons or metres and litres.
- Enter the room's length, width and wall height.
- Add the number of doors, windows and coats, and choose whether to paint the ceiling.
- Read how much paint you need and how many whole cans to buy.
How it works
The wall area is the distance around the room multiplied by the wall height. The calculator then takes off a typical door and window, multiplies by the number of coats and divides by the paint's coverage.
Most interior wall paint covers about 350 to 400 square feet per gallon, or roughly 10 m² per litre, on smooth walls. Textured walls, bare plaster and new drywall soak up more, so expect lower coverage or use a primer first. Two coats are normal, especially when changing colour. A US gallon is 3.785 litres, and a quart is a quarter of a gallon.
Formula
Wall area = 2 × (length + width) × height − doors − windows Paint = (wall area + ceiling) × coats ÷ coverage
Example
A 14 × 12 ft bedroom with 8 ft walls, one door and two windows has about 366 sq ft of wall. Two coats at 350 sq ft per gallon need 2.09 gallons, so buy 2 gallons and a quart, or 3 gallons. In metric, a 4.2 × 3.6 m room with 2.4 m walls has about 32.7 m² of wall and needs about 6.5 litres for two coats.
Paint for common room sizes
Walls only, 8 ft ceilings, one door, two windows, two coats at 350 sq ft per gallon.
| Room | Wall area | Paint | Buy |
|---|---|---|---|
| 10 × 10 ft | 270 sq ft | 1.5 gal | 2 gal |
| 12 × 12 ft | 334 sq ft | 1.9 gal | 2 gal |
| 14 × 12 ft | 366 sq ft | 2.1 gal | 3 gal |
| 16 × 14 ft | 430 sq ft | 2.5 gal | 3 gal |
| 20 × 15 ft | 510 sq ft | 2.9 gal | 3 gal |
Frequently asked questions
How much paint do I need for a 12 × 12 room?
About 1.9 gallons for two coats on 8 ft walls with one door and two windows, so buy 3 gallons, or 2 gallons and a quart.
How many coats of paint do I need?
Two coats for most walls. One coat can be enough when repainting the same colour, while bright or dark colours over a light wall may need three, or a tinted primer.
How much paint do I need per square foot?
About 1 gallon for every 350 to 400 square feet per coat. Divide your wall area by the coverage on the can.
Should I buy extra paint?
Keep a little left over for touch-ups. Buying from one batch also avoids slight colour differences between cans.
Last reviewed October 21, 2026. Spotted an error? Tell us.