Why people always buy the wrong amount
Paint is one of those purchases where guessing feels harmless and costs real money. Buy too little and you are back at the shop mid-job, hoping the second tin matches the first batch — it often does not, quite. Buy too much and an expensive half-tin sits in the garage for three years until it goes off. The fix is ten minutes of measuring and one simple calculation.
Step 1: Measure the walls
For each wall, measure its width and its height in metres, then multiply. A wall 4 metres wide and 2.7 metres high is 10.8 square metres. Add up all the walls you plan to paint.
Worked example — a bedroom with four walls:
- Wall 1: 4.0 × 2.7 = 10.8 sq m
- Wall 2: 4.0 × 2.7 = 10.8 sq m
- Wall 3: 3.5 × 2.7 = 9.45 sq m
- Wall 4: 3.5 × 2.7 = 9.45 sq m
- Total wall area: 40.5 sq m
If the room is a simple rectangle, there is a shortcut: perimeter × height. Perimeter = 2 × (length + width) = 2 × (4.0 + 3.5) = 15 metres. 15 × 2.7 = 40.5 sq m. Same answer, less arithmetic.
Step 2: Subtract doors and windows
Paint does not go on doors and windows, so subtract them:
- A standard door is about 2.0 × 0.8 = 1.6 sq m
- A typical window is about 1.2 × 1.2 = 1.44 sq m
Our bedroom has one door and one window: 40.5 − 1.6 − 1.44 = 37.46 sq m of paintable wall. Call it 37.5 sq m. Skipping this step overestimates by about 8% — roughly the cost of an extra litre.
Step 3: Divide by coverage, then double for two coats
A tin’s label states its coverage, usually around 10–12 sq m per litre for wall paint. One coat for our room: 37.5 ÷ 11 = 3.4 litres. But almost every wall needs two coats for an even finish, so double it: 6.8 litres. Round up to the next tin size — 7.5 litres, or two 5-litre tins if that is how your shop sells it.
The formula in one line: Litres = (Wall area ÷ Coverage per litre) × Number of coats, rounded up.
The things that change the answer
Dark colours over light walls. Covering a dark red wall with white paint can take three coats, not two. Budget an extra coat whenever the colour change is dramatic.
Rough or new plaster. Fresh plaster drinks paint. A mist coat (watered-down first coat) is standard on new plaster, and it adds roughly half a coat’s worth of paint to the total.
Ceilings. If the ceiling is being painted too, add its area: length × width. Our bedroom ceiling is 4.0 × 3.5 = 14 sq m, which at two coats adds about 2.5 litres.
Wastage. Rollers, trays, and the paint that stays in the tin account for roughly 10%. Rounding up to the next tin size usually covers this — another reason to round up, never down.
A second worked example
A living room, 5 m × 4 m, height 3 m, two doors, two windows, two coats, coverage 12 sq m per litre.
- Perimeter: 2 × (5 + 4) = 18 m. Wall area: 18 × 3 = 54 sq m.
- Subtract: two doors 3.2 + two windows 2.88 = 6.08. Paintable: 47.92 sq m.
- One coat: 47.92 ÷ 12 = 4.0 litres. Two coats: 8.0 litres. Buy 10 litres (two 5-litre tins) to cover wastage and touch-ups.
Questions people actually ask
Can I just buy one big tin and be done?
If the calculation says 7 litres and the shop sells 10-litre tins, yes — one tin is cheaper than two small ones, and leftover paint is useful for touch-ups. Just seal it properly and store it upright.
Does the paint finish change coverage?
Matte emulsion usually covers best. Gloss and silk finishes can need slightly more, and textured surfaces always need more. Check the tin — the stated coverage is measured for that exact product.
What if two tins are different batches?
Mix them together in a bucket before painting (“boxing” the paint). Two tins of the “same” colour from different batches can differ slightly, and the difference shows as a line halfway down the wall.
How do I store leftover paint?
Clean the rim, tap the lid shut firmly, label it with the room and date, and store it somewhere cool and frost-free. Good emulsion keeps for two to three years; if it smells sour when opened, throw it away.