Paint calculator: how much paint you need for a room

Add up the length of the walls and multiply by the wall height. Take off about 20 sq ft (1.9 m²) for each door and 15 sq ft (1.4 m²) for each window, Behr's averages, then multiply by the number of coats and divide by the coverage on the can. A 4 × 3.5 m room with 2.4 m walls, one door and one window has 32.7 m² to paint; two coats at 10 m² per liter take 6.5 liters, so buy a 5 L and a 2.5 L can.

With 8 ft walls, one door and one window, a 12x12 room has 349 sq ft of wall to paint. Two coats at 400 sq ft per gallon take about 1.75 gallons, so buy 2 gallons.

Multiply the room's length by its width. A 12x12 ceiling is 144 sq ft, and two coats at 400 sq ft per gallon take about three-quarters of a gallon, so buy 1 gallon. Check “Paint the ceiling too” to get it as a separate total, since ceilings are often painted with a different paint.

Sherwin-Williams says a gallon typically covers about 350 to 400 sq ft, and Glidden says up to 400 sq ft on primed, smooth walls. Dulux lists up to 14 m² per liter for Dulux Matt and up to 10 m² for its one-coat paint. The calculator starts at 400 sq ft per gallon and 10 m² per liter; change it to the figure on your can.

Plan on two coats for walls; that's where the calculator starts, and it's what Dulux recommends for Dulux Matt. A strong color change, bare plaster or drywall, and rough or porous walls can take more paint. A one-coat paint over a similar color may need just one.

It adds 10% for touch-ups unless you change it, then covers that total with as little left over as it can, without swapping one bigger can for a pile of small ones. It uses 1, 2.5, 5 and 10 liter cans, or quarts, gallons and 5-gallon buckets. Sizes vary by brand, so check what your store sells.

Check your lease and ask your landlord in writing before you paint. If you can't paint, Roomcast shows how your room could look in a new style with the walls left as they are.