Paint calculator
Room size to litres on the shelf: wall area with the openings taken out at standard sizes, multiplied by coats, divided by the tin's spreading rate.
Opens the editor with this room already drawn at your wall height. Add the real doors and windows and every wall area stops being a standard-size guess.
The formula
perimeter × height − openings, then × coats ÷ spreading rate.
For the defaults: a 4.2 × 3.6 m room has a 15.6 m perimeter, so
15.6 × 2.7 = 42.1 m² of wall. One door (−1.89 m²) and one window
(−1.44 m²) leave 38.8 m² to paint. Two coats at 10 m²/L is
38.8 × 2 ÷ 10 = 7.8 L — call it
8 litres.
The opening sizes are the same standards the editor draws by default — see standard door sizes and standard window sizes for where 900 × 2100 and the 900 mm sill come from.
Common questions
Related: the flooring calculator for the same room's floor, and the U-value calculator if the real question is what's inside the wall.
How many square metres does a litre of paint cover?
Most wall emulsions state 10–12 m² per litre per coat on the tin; that is the spreading rate this calculator defaults to (10). Rough or freshly plastered surfaces drink more — drop to 8. The tin's own figure always wins: it is measured for that paint.
Do I really need two coats?
Almost always yes for a colour change, and even white-on-white usually shows through one coat under raking light. One coat is realistic only when repainting the same colour on a sound surface. A strong colour over a dark one may take three — or one tinted primer plus two.
How does the calculator handle doors and windows?
Each door subtracts 1.89 m² (a standard 900 × 2100 mm opening) and each window 1.44 m² (1200 mm wide, sill at 900 mm, head at 2100 mm) — the same standard sizes the Gridsnap editor draws by default. If your openings are unusual — a full-height window wall, double doors — draw the room and read the real wall areas instead.
What about the ceiling?
Tick "include ceiling" and the room's floor area is added to the painted area — a flat ceiling has the same area as the floor. Ceilings are usually a different paint, so run the calculator twice if you want the quantities separately: once walls-only, once ceiling-only (set doors, windows and one wall pair to zero, or just multiply width × length by coats ÷ spreading rate).
Litres or tins — what do I buy?
The calculator rounds up to the nearest half litre for a shopping figure. Tins come in 1, 2.5, 5 and 10 L; buying one size up beats running out mid-wall, because a new batch can dry a shade different. Keep the leftover labelled for touch-ups.
Flooring & Tile Calculator · Stair Calculator · Floor Plan Scale Calculator · U-Value Calculator
Stop estimating wall areas
Draw the room once and the plan holds every wall length and opening size — repaint quantities become a lookup, not a ladder-and-tape job.