Paint Calculator: How Many Litres or Gallons You Need
Paint quantity comes from wall area divided by the coverage rate on the tin, multiplied by the number of coats. Most emulsions cover about 10-12 m² per litre (roughly 350-400 ft² per gallon) on a smooth, previously painted surface.
Measuring the surface
Multiply the perimeter of the room by the wall height for total wall area, then subtract doors (about 1.8 m² each) and windows (about 1.5 m² each). Include the ceiling separately if you are painting it. Ignoring openings on a small room can leave you with a full unopened tin.
Coverage drops on difficult surfaces
- Bare plaster and new drywall absorb heavily — expect 20-30% less coverage and use a mist coat first.
- Textured or masonry surfaces can cut coverage by up to half.
- Dark to light colour changes usually need three coats or a tinted primer.
- Sprayed application uses more paint than a roller; brushing uses the least.
Buy in one batch
Tinted paint varies slightly between production batches, and the difference shows on a large wall. Buy all the paint for a room at once, check the batch numbers match, and box multiple tins together in a larger bucket before starting so the colour is uniform from the first cut-in to the last roll.
Key takeaways — Paint Calculator
- Litres = (wall area − openings) × coats ÷ coverage per litre.
- Two coats is the realistic default; three for strong colour changes.
- Prime bare plaster, and buy all tins from the same batch.