Measure the walls, choose your coats and use the coverage on your paint tin. We’ll turn those numbers into gallons and litres, with primer kept separate.
Painting the ceiling too? Switch it on below. Doors and windows use standard allowances, so adjust your measurements if your openings are unusually large.
How to use this calculator
- Measure the length and width of the room at floor level, and the height from floor to ceiling.
- Count the doors and full-size windows — the calculator subtracts a standard area for each.
- Choose the number of coats. Pick two for most repainting jobs, three over dark colours.
- Press calculate, then round up to the next full gallon or tin size your store sells.
How paint quantities are calculated
Paintable area = the wall perimeter × height, minus the opening allowances, plus the ceiling if selected. Paint gallons = paintable area × coats ÷ your paint coverage. Primer is calculated separately for one coat using its own coverage. Round the paint order up to a whole gallon, and confirm the tin sizes sold by your supplier.
For a 12 × 10 ft room with 8 ft walls, one standard door and two standard windows, the net wall area is 301 sq ft. Two coats at 350 sq ft per gallon need about 1.72 gallons: round up to 2 gallons. One primer coat at 250 sq ft per gallon needs about 1.20 gallons.
A few things worth checking
- Forgetting the second coat in the estimate, then running out halfway through the final wall.
- Ignoring doors and windows entirely, which can overstate the area of a small room by 15 percent or more.
- Using the 400 sq ft per gallon figure printed for ideal conditions — real rooms with texture or repairs get closer to 300 to 350.
Your questions, answered
How much paint do I need for a 12x12 room?
For walls only with 8 ft ceilings, one door, two windows and coverage of 350 sq ft per gallon, the tool estimates about 1.90 gallons for two coats, rounded up to 2 gallons. Enter your actual measurements and the coverage on your chosen paint.
Should I buy extra paint for touch-ups?
The tool rounds the finish-paint estimate up to a whole gallon. That rounding may leave some spare paint, but it does not guarantee a particular amount for repairs. Keep the product, colour and batch details with your project notes, and ask your supplier about a suitable touch-up allowance.
Does primer count as a coat of paint?
This tool calculates primer separately from the finish paint, using one primer coat and its own coverage field. It does not decide whether your surface needs primer or how many finish coats are suitable; use the instructions for the paint system you choose.
What if my room is not rectangular?
Break the work into rectangular sections and add their unrounded quantities before deciding what to buy. Count each opening once. For unusual wall shapes or openings, check the actual paintable area rather than relying only on the standard door and window allowances.
How to Measure a Room for Paint Without Overbuying
A tape measure, a few notes and the right coverage figure are all you need to make a sensible paint order.