Comprehensive Calculator Guide
Paint needed, in litres, is the wall area × number of coats ÷ the area one litre covers, plus a small allowance. A 4 × 5 m room 2.8 m high, less a door and a window, needs about 10.3 litres for two coats at 10 m² per litre, or three 3.6-litre cans. Enter area, coats and coverage to see the amount instantly.
Measure the walls first
Wall area is the room's perimeter times its height. A 4 × 5 m room has an 18 m perimeter, so at 2.8 m high its walls cover 50.4 m². Subtract openings: a 2 m² door and a 1.5 m² window leave 46.9 m² to paint. Work out the ceiling separately (length × width), as it usually takes a different paint.
Coverage per litre per coat is printed on the can or in the product data sheet; many interior wall paints cover roughly 8–12 m² per litre, but use your product's figure. New plaster and rough surfaces absorb more and need a primer coat, calculated separately.
From litres to cans
Paint is often sold in 3.6-litre cans, plus 18-litre and 1-litre sizes. Divide the litres by the can size and round up: 10.3 ÷ 3.6 = 2.9, so three cans. Allow 5–10% for paint left in trays and rollers, and buy a custom-tinted colour in one go, since a second mix can differ slightly.
How to Use
- 01Work out net wall area (perimeter × height minus doors and windows) and enter it in m².
- 02Enter the number of coats, usually two over a primer.
- 03Enter coverage per litre from the can.
- 04Add an allowance, then divide the litres by your can size.
Formula Used
Litres = area × coats ÷ coverage per litre × (1 + allowance ÷ 100)Practical Examples
Example 1: walls of a 4 × 5 m room
46.9 m², two coats, 10 m²/L, 10% allowance: 10.32 litres, three 3.6-litre cans.
Example 2: the ceiling
20 m², two coats, 10 m²/L, 10% allowance: 4.4 litres.
Example 3: a primer coat
60 m², one coat, 12 m²/L, 5% allowance: 5.25 litres.
Important Tips
- Expect an extra coat when covering a dark colour with a light one.
- Keep leftover paint labelled with its colour code for touch-ups.
Common Mistakes to Avoid
- Using the floor area instead of the wall area.
- Forgetting to multiply by the number of coats.
Frequently Asked Questions
- How much paint for a 4 × 4 m room?
- At 2.8 m high the walls are 44.8 m²; less 3.5 m² of openings that is 41.3 m². Two coats at 10 m²/L with 10% extra is about 9.1 litres.
- How far does a litre of paint go?
- It depends on the product and surface; many interior wall paints cover about 8–12 m² per coat. Use the figure on your can.
- How many coats do I need?
- Usually two finish coats, over a primer on new walls, and sometimes a third over a dark colour.
- Should I subtract doors and windows?
- Yes, though small openings can be left in as part of the allowance.