Enter your room size, the area taken up by doors and windows, the number of coats and the coverage printed on your paint can. The calculator tells you how much paint to buy, rounded up to whole gallons or litres. The ceiling is only included if you tick the box.
This calculator needs JavaScript. The formula and a worked example are below.
How to use the paint calculator
Measure the length and width of the room and the wall height. Add up the area of every door and window. A standard door is roughly 20 sq ft, and a medium window is often around 15. Then read the coverage on your chosen paint. Cans often state a figure in the region of 350 to 400 sq ft per gallon, but the label on your product is the number to trust.
The formula
Wall area = 2 × (length + width) × height. Paintable area = wall area − openings. Paint needed = paintable area × coats ÷ coverage × (1 + waste). If the ceiling is included, its area (length × width) is added before multiplying.
Worked example
A 12 ft by 10 ft room with 8 ft walls has 352 sq ft of wall. Take off 40 sq ft for a door and two windows to get 312 sq ft. Two coats at 350 sq ft per gallon need 1.78 gallons, and 10% waste brings that to 1.96 gallons. Buy 2 gallons. Add the ceiling and the answer rises to 3 gallons.
What affects how much you really use
Rough or unpainted drywall soaks up more paint, and a large colour change often needs an extra coat. Dark colours over light walls, and the reverse, are the usual cause of running short. A primer coat is separate and does not count towards your topcoat total.
Textured walls also use more than smooth ones. If you are unsure, keep the waste allowance at 10% or a little higher and buy a spare small can for touch-ups.
Frequently asked questions
Does this include primer?
No. Run the calculator once with one coat and the primer’s coverage figure to estimate primer separately.
Should I paint trim and doors too?
Trim is usually painted in a different finish, so treat it as its own small job. Doors are included here as openings, which means they are not counted.
Why round up?
Paint is sold in whole cans. Matching a colour later is harder than buying a little extra now.
Related guides
Choosing a colour first? Start with interior paint colors. Painting a room that will also need new flooring or trim? Try the flooring calculator and the baseboard calculator.