Paint Calculator — How Much Paint for a Room
Work out how many gallons or litres of paint you need for a room’s walls and ceiling, after doors and windows, for any number of coats.
Estimate how much paint a room needs before you go to the store. Enter the room’s length, width and wall height, count the doors and windows, and choose the number of coats. The calculator subtracts the openings, adds the ceiling if you want it, and converts the area into gallons and quarts, or litres, using the coverage printed on your paint can.
How to use this tool
- Choose feet and gallons or metres and litres.
- Enter the room length, width and wall height.
- Count the doors and windows, and adjust their sizes if they are unusual.
- Set the number of coats and the can’s coverage, then read how much paint to buy.
Formula
Paint = (2 × (L + W) × H − openings + ceiling) × coats ÷ coverage
L and W are the room’s length and width and H is the wall height. Openings default to 20 sq ft (1.9 m²) per door and 15 sq ft (1.4 m²) per window. Interior paint typically covers about 350–400 sq ft per US gallon, or roughly 10–12 m² per litre; the exact figure is on the label.
Example
A 14 × 12 ft bedroom with 8 ft walls has 2 × (14 + 12) × 8 = 416 sq ft of wall. Taking off one door (20 sq ft) and two windows (30 sq ft) leaves 366 sq ft. Two coats make 732 sq ft, and at 350 sq ft per gallon that is 2.09 gallons, so buy 3 gallons, or 2 gallons plus 1 quart. Adding the ceiling brings it to 3.05 gallons.
Frequently asked questions
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door and two windows, the walls are 334 sq ft after openings. Two coats need about 1.9 gallons, so 2 gallons is enough.
Do I need two coats of paint?
Usually, yes. Two coats give even colour and better durability. One coat can work when repainting the same colour with a high-hide paint.
Should I subtract doors and windows?
Yes, for a closer estimate, but keep a little extra. Leftover paint is useful for touch-ups.
Last updated: 2026-10-30