Buying too little paint means an extra trip to the shop mid-job — and the risk that the new tin is a slightly different shade. Too much is money wasted. A quick calculation before you start means you buy exactly the right amount.
Step 1: Calculate the paintable surface area
Measure the walls you want to paint. For each wall: height × width. Add them together for the total area.
Then subtract doors and windows — they don't need painting.
Example: 36.4 − 1.8 − 1.2 = 33.4 m²
Step 2: Check the coverage on the tin
Every tin of paint states its coverage (spreading rate) in m² per litre. Typical values:
| Paint type | Coverage per litre |
|---|---|
| Standard wall paint | 10–12 m² |
| Matt emulsion | 8–10 m² |
| Primer / undercoat | 6–10 m² |
| Gloss / satin | 12–16 m² |
| Masonry paint (exterior) | 4–8 m² |
Step 3: Factor in the number of coats
Most jobs require at least 2 coats for even coverage — especially when going from a dark colour to a lighter one, or painting a new wall. For a full colour change, 2–3 coats is normal.
Step 4: Adjust for surface type
Not all surfaces absorb paint the same way. Porous, rough or unpainted surfaces need more paint than smooth, previously painted walls.
| Surface | Adjustment |
|---|---|
| Smooth, previously painted | No adjustment needed |
| New plaster or drywall | Add 20–25% extra |
| Rough or textured surface | Add 15–20% extra |
| Bare wood | Add primer coat first |
| Dark colour to light | Add 1 extra coat |
💡 Always round up — and keep the leftovers
Round up to the nearest available tin size. Keep any leftover paint sealed and labelled — you'll thank yourself when you need to touch up a scuff or mark down the line. Paint from the same batch matches perfectly.
Full example
Painting a living room: 4 walls, room is 5 m × 4 m × 2.7 m high. One door, two windows. Standard matt emulsion (10 m²/L). 2 coats.
- Wall area: (5×2.7)×2 + (4×2.7)×2 = 27 + 21.6 = 48.6 m²
- Minus openings: 48.6 − 1.8 − (2×1.2) = 44.4 m²
- Litres: (44.4 ÷ 10) × 2 = 8.88 L
- Buy: 3× 4L tins (12L) — enough for 2 full coats plus some spare
🎨 Calculate paint automatically with BuildExact
Enter your room dimensions, select the paint type and number of coats — BuildExact calculates exactly how many litres you need. Save it as a project or export as PDF.
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