Running out of paint halfway through a wall is one of the most avoidable DIY frustrations. Buy too much and you waste money; buy too little and you risk a visible seam where the new can meets the old. The good news is that estimating paint is simple arithmetic once you know the steps.
The short answer
To estimate paint, you need four things: the area of the walls, the area of the doors and windows to subtract, the number of coats, and the coverage rating printed on the can. The formula is: paint = (wall area − openings) × coats ÷ coverage. Add about 10% for waste and touch-ups, and round up to whole cans.
Step 1: Measure the walls
Treat the room as four walls. Measure the perimeter — for a rectangular room use 2 × (length + width). Multiply the perimeter by the ceiling height to get the gross wall area. A 5 m × 4 m room with 2.5 m ceilings has a perimeter of 18 m and a gross wall area of 45 m².
Step 2: Subtract doors and windows
You do not paint doors and windows, so subtract them. A standard door is about 1.9 m² (around 21 ft²) and an average window about 1.4 m² (around 15 ft²). One door and two windows remove roughly 4.7 m², leaving about 40 m² of paintable wall.
Step 3: Decide on coats
Most jobs need two coats for an even, durable finish. You may get away with one when repainting a similar colour, but expect two when covering a darker shade or fresh filler.
Step 4: Check the coverage
Every can lists a coverage figure — typically 10–12 m² per litre (about 350 ft² per gallon) for smooth, previously painted walls. Rough or bare surfaces drink more, so lean to the lower end.
A worked example
Paintable area 40 m² × 2 coats = 80 m² to cover. At 11 m² per litre, that is about 7.3 litres. Add 10% for waste and you land near 8 litres — two 4-litre cans, or one 5-litre plus a top-up.
Common mistakes to avoid
- Forgetting the second coat — it roughly doubles your paint.
- Using smooth-wall coverage on bare plaster, which absorbs far more.
- Buying exact amounts with no margin for touch-ups.