Measure the walls
Add the lengths of all the walls to get the perimeter and multiply by the height. A room 4 m × 3.5 m with walls 2.5 m high has 2 × (4 + 3.5) × 2.5 = 37.5 m² of wall.
Take off doors and windows
A standard door is about 1.9 m² (21 ft²) and a window about 1.4 m² (15 ft²). With one door and two windows the room above has 37.5 − 1.9 − 2 × 1.4 = 32.8 m² to paint.
Coats and coverage
The tin gives the coverage, often about 10 m² per liter or 350 to 400 ft² per US gallon. Two coats are normal, and more are needed for a big color change or bare plaster.
32.8 m² × 2 coats ÷ 10 m²/L = 6.6 liters. Buying a 5 L and a 2.5 L tin covers it with a little spare for touch-ups.
Tips
- Measure the ceiling separately (length × width) if you are painting it too.
- Rough or porous surfaces absorb more paint than the tin says.
- Buy all the paint for one room at once so the color matches.
Calculator
The Paint Calculator does the whole calculation in meters or feet. For the floor, the Flooring Calculator works out the boxes to buy.
The same room in feet
A room 12 ft × 14 ft with 8 ft walls has 2 × (12 + 14) × 8 = 416 ft² of wall. Take off one door (21 ft²) and two windows (30 ft²) to leave 365 ft². Two coats is 730 ft², and at 375 ft² per gallon that is 1.95, so 2 gallons. The ceiling is 12 × 14 = 168 ft², and two coats need about 0.9 gallons, so one more gallon of ceiling paint.
When to use primer
- New plaster or drywall, which soaks up the first coat unevenly.
- A big change from dark to light.
- Stains, such as water marks or smoke, which can bleed through ordinary paint.
- Filled patches, which otherwise show as dull spots in the finish.
Common problems
- The color looks different between cans. Small differences between batches show on a large wall. Pour all the cans into one bucket and mix them before starting.
- Patchy shine after drying. Filled areas and bare spots absorb paint differently. Prime them, then paint the whole wall.
- The paint ran out early. Textured walls and rollers with a long pile use more paint than the coverage on the tin. Allow 10 to 20% extra on rough surfaces.