Paint Calculator: How Much Paint Do I Need?
Enter your room's length, width, and wall height, choose how many coats you want, and tell the calculator how many doors and windows to skip. It returns the litres you need, adds a 10 % waste buffer, and tells you exactly how many 2.5 L and 5 L tins to put in your trolley.
Paint needed (with 10% buffer)
~8.2 L
2.5 L tins
4
5 L tins
2
Area breakdown & assumptions
| Surface | Area (m²) |
|---|---|
| Walls | 40.50 |
| Ceiling | 0.00 |
| Openings (−) | 3.34 |
| Paintable total | 37.16 |
| Litres (no waste) | 7.43 |
Assumptions: standard door 0.8 × 2.4 m ≈ 1.9 m²; standard window 1.2 × 1.2 m = 1.44 m²; waste factor +10 % (tray residue, roller nap, touch-ups). Cans are rounded up (ceiling).
How it works
The total paintable area is built in three steps. Wall area is 2 × (length + width) × height — the four walls treated as a rectangle. Ceiling area is length × width, added only when you tick the ceiling option. Openings are subtracted last: each standard door counts as 0.8 m × 2.4 m ≈ 1.9 m², each standard window as 1.2 m × 1.2 m = 1.44 m². The result is clamped to zero so an unusually door-heavy room never yields negative paint.
Coverage varies considerably by paint type and surface condition. A premium one-coat emulsion may cover 14–16 m² per litre; a budget contract white might manage only 6–8 m². Porous new plaster and bare brick absorb far more paint than a previously painted surface in good condition. The coverage knob in this calculator lets you dial in the figure from your specific tin — check the back label. When in doubt, use a conservative number (lower coverage = more litres = safer estimate).
A 10 % waste buffer is baked in because real painting always involves some loss: tray residue, roller nap absorption, touch-ups, and the fact that a half-empty tin gets thrown away. The calculator shows you both the theoretical litres and the waste-inclusive figure, then rounds the can counts up with ceiling arithmetic — because you cannot buy 0.3 of a tin.
Frequently asked questions
One coat or two?+
Most professional painters use two coats as a baseline: the first builds coverage and reveals holidays; the second delivers the even finish and the full listed colour. One coat is sometimes sufficient for a minor colour change on a smooth, previously painted surface with a high-opacity paint. Going darker or lighter by more than a shade almost always needs two coats, and white-over-dark typically needs three. Set the coats field to match your plan, not your hope.
Do I subtract doors and windows?+
Yes — the calculator subtracts them automatically using standard sizes (door 0.8 × 2.4 m, window 1.2 × 1.2 m). If your openings are unusually large or small, adjust the coverage figure rather than fighting the count: a lower coverage value accounts for extra cutting-in time and roller transitions around larger openings; a higher value offsets the over-subtraction of very small windows. For a room with many large patio doors, you may prefer to set doors to zero and reduce coverage instead.
Does this include primer?+
No. Primer is a separate product with different coverage rates and is almost always a separate purchase. On bare plaster, new drywall, or heavily patched surfaces you should prime first — typically at 1 coat with a dedicated primer, then add your topcoat count on top. Run the calculator twice: once with primer coverage to size the primer order, then again with your topcoat coverage for the finish coats.