How it is calculated
Walls = 2 × (length + width) × height. Doors and windows are subtracted and, if chosen, the ceiling (length × width) is added.
Paint = net area × coats ÷ coverage × (1 + margin). Tins are always rounded up.
Worked example
Using the calculator’s example values:
- How do you want to enter the area?: Room dimensions
- Room length: 4 m
- Room width: 3 m
- Wall height: 2.5 m
- Paint the ceiling too: No
- Number of doors: 1
- Area of each door: 1.6 m²
- Number of windows: 1
- Area of each window: 1.5 m²
- Number of coats: 2
- Paint coverage: 10 m²/L
- Safety margin: 10%
- Tin size: 4 L
Result: Paint needed — 7.02 L (tins to buy: 2 units; net area to paint: 31.9 m²; total area across all coats: 63.8 m²).
Money-saving tips
- Prime new or very porous walls first.
- Buy all tins from the same batch to avoid colour differences.
- Keep a little paint for future touch-ups.
Frequently asked questions
How many coats do I need?
Two is typical. Strong colour changes or new walls may need a third coat or a primer.
What coverage does paint have?
The manufacturer prints it on the tin, as area per litre or gallon per coat. It varies with paint quality and the wall’s texture and porosity.
Should I subtract doors and windows?
Yes, if they are large. If you also paint the frames, reduce their area in the calculator.
How we calculate and the limits of the results: Methodology.