Work out how many litres of paint a room needs and which cans to buy — right in your browser.
Buying paint by eye is how a job ends with half a bucket left over or a second trip to the shop mid-wall. This calculator does the arithmetic a painter does on the back of an envelope: it turns the room into wall area, subtracts the openings, multiplies by the number of coats, divides by the coverage the manufacturer prints on the can and adds a margin for what the roller, the tray and the touch-ups will swallow. A 4 × 3 m bedroom with a 2.7 m ceiling, one door and one window needs 34.7 m² painted twice — 6.94 litres at 10 m²/L, or 7.6 litres once the 10% waste margin is in.
The wall area comes from the perimeter times the ceiling height, and the perimeter from 2 × (width + length); if the room is not rectangular you can type the perimeter directly instead. Openings are discounted at 1.6 m² per door and 1.5 m² per window by default, both editable, since a sliding glass door or a small bathroom window will not match those. Every other input is a lever: coats, coverage and the waste margin all stay in your hands. The can suggestion is a small optimisation rather than a rounding — it searches combinations of the three standard Brazilian sizes and picks the cheapest one that still covers the volume, assuming an 18 L bucket costs roughly half per litre of what a 0.9 L can does, which is why it sometimes suggests a bucket with litres to spare.
The number that most often breaks a paint estimate is the coverage. The figure on the label is measured on a sealed, smooth surface in laboratory conditions; raw plaster, old chalky paint and heavy texture can drink 30% to 50% more, and covering a dark colour with a light one usually takes three coats, not two. Ceilings are not included here — run the calculation again with the ceiling area if you are painting it — and neither are primer, sealer or the putty you will end up buying. Everything runs in JavaScript on your device, so nothing you type about your home leaves the page.
Wall area = perimeter × ceiling height, where the perimeter is 2 × (width + length). Subtract the openings: doors × 1.6 m² and windows × 1.5 m² by default. Litres = wall area × coats ÷ coverage in m²/L, then × (1 + waste margin ÷ 100). The can suggestion searches combinations of 18 L, 3.6 L and 0.9 L and keeps the cheapest one that covers that volume, assuming an 18 L bucket costs about half per litre what a 0.9 L can does.
Multiply the wall perimeter by the ceiling height, subtract the doors and windows, multiply by the number of coats and divide by the coverage on the can. A 4 × 3 m room with a 2.7 m ceiling, one door and one window needs about 6.9 litres for two coats at 10 m²/L, or 7.6 litres with a 10% waste margin.
Around 10 m² per litre per coat for a standard interior latex, but always check the label — coverage varies widely by product. Porous, unsealed or heavily textured walls can need 30% to 50% more than the nominal figure.
Two is the default and covers most repaints in a similar colour. Go to three when you are painting a light colour over a dark one, covering strong stains, or working on a wall that has never been painted.
Because the 18 L bucket costs roughly half per litre of the small cans, so it is often cheaper even with paint left over. The suggestion balances volume against that price difference rather than just rounding up.
No — the calculation covers walls only. To paint the ceiling too, run it again using the ceiling area (width × length) and add the two results.
No. Everything is calculated in JavaScript on your device. Nothing you type is transmitted or stored.
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