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 — add the length of all the walls together, or 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. As a rule of thumb, 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 in our example.
Step 3: Decide on coats
Most jobs need two coats for an even, durable finish. You may get away with one coat when repainting a similar colour, but expect two when covering a darker shade, a bold colour, or fresh filler. Painting over a strong colour with a pale one can even need a primer plus two coats.
Step 4: Check the coverage
Every can lists a coverage figure — typically around 10–12 m² per litre (about 350 ft² per gallon) for a smooth, previously painted surface. Rough, porous or bare surfaces drink more paint, so lean toward the lower end of the range if your walls are new plaster or textured.
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 — so two 4-litre cans, or one 5-litre plus a small top-up. Rounding up to whole cans is always the safe move.
Common mistakes to avoid
- Forgetting the second coat — it roughly doubles your paint needs.
- Using the coverage for smooth walls on bare plaster, which absorbs far more.
- Buying exact amounts with no margin — always keep a little for touch-ups later.
- Mixing cans mid-wall instead of boxing (combining) them first for a consistent colour.