How to work out how much paint you need
- Find the wall area. Add up the length of every wall (the room’s perimeter) and multiply by the wall height.
- Subtract the openings. A standard door is about 21 sq ft (2 m²) and a typical window about 15 sq ft (1.4 m²).
- Multiply by the number of coats.
- Divide by the coverage on the can. Most interior paints list 250–400 sq ft per gallon, or 8–12 m² per liter.
Worked example: a 12 × 12 ft bedroom with 8 ft ceilings
Perimeter is 48 ft, so the walls are 48 × 8 = 384 sq ft. Take off one door (21 sq ft) and two windows (30 sq ft) to get 333 sq ft. Two coats is 666 sq ft. At 350 sq ft per gallon that is 1.9 gallons, so buy 2 gallons.
Gallons or quarts?
Quarts cost more per ounce than gallons. Once you would need three quarts, a gallon is usually the same price or cheaper, so the calculator rounds up to a gallon at that point. For metric tins it picks the combination of 5 L, 2.5 L and 1 L tins that covers the job with the least leftover paint.
When coverage is lower than the can says
The figure on the can assumes a smooth, primed, similar-colored wall. Plan for more paint when:
- The surface is textured, rough plaster or bare masonry. These soak up paint.
- You are covering a dark color with a light one. Add a coat, or prime first.
- The wall is new drywall or patched. Prime it so the finish coat dries evenly.
- You are using a roller with a thick nap. It holds and spreads more paint.
Ceilings and trim
A ceiling is simply length × width. Ceilings usually take flat ceiling paint, so tick the ceiling option only if you will use the same paint. Trim, doors and baseboards are usually painted in semi-gloss and need much less: a quart covers the trim of an average room.
Papering instead? The wallpaper calculator counts rolls by strips, which is more accurate than dividing by area.
Keep some leftover
Keep a little of each color, labeled with the room and the date. Touch-ups match far better from the same batch than from a new can mixed to the same formula.