How it is calculated
The calculator adds up the four walls (perimeter × height), subtracts a standard door (1.6 m²) and window (1.8 m²) for each opening, then multiplies the area by the number of coats and divides by the coverage printed on the can. A reserve is added for rollers, trays and uneven walls. Finally it picks the combination of cans that gives the smallest leftover.
Wall area = 2 × (length + width) × height − doors × 1.6 − windows × 1.8Litres = area × coats ÷ coverage × (1 + reserve)Example
A room of 4 × 3 m with a 2.6 m ceiling, one door and one window has 33 m² of walls. Two coats at 10 m²/L plus 10 % reserve need 7.3 L: one 5 L can and one 2.5 L can.
Tips from practice
- Fresh plaster and filler absorb more paint. Prime first, or count one extra coat.
- A strong colour change, such as white over dark blue, usually needs three coats.
- Buy all cans of a tinted colour at once: shades from different batches can differ.
Questions and answers
How many m² does one litre of paint cover?
Most interior wall paints cover 8–12 m² per litre per coat on a smooth primed wall. The exact figure is printed on the can; enter it in “Coverage per coat”.
Should I subtract doors and windows?
Yes, the calculator does it for you: 1.6 m² per door and 1.8 m² per window. For very large windows, switch to “Wall area” and enter your own measurement.
Why does the result include a reserve?
Some paint stays in the roller, tray and brush, and rough walls take more. 10 % is a practical minimum; you can change it.
Found a mistake in a formula? Write to contact@planicalc.com