How much paint you need
Enter the room size, the number of coats and the paint coverage: get the liters needed.
How it works
The surface to paint is the room perimeter times the height, minus openings. Liters depend on the coats and the coverage on the tin (usually 8-12 m²/L per coat).
Tip
Always buy a little extra: keeping the same batch avoids shade differences on touch-ups.
The coverage printed on the tin is optimistic
The eight to twelve square metres per litre per coat hold on a wall that is smooth, already painted and the same colour. On fine render it drops, on bare or porous plaster the paint is drunk and it can halve. It is why the first coat on a new wall uses almost twice as much as the second: the surface absorbs until it is saturated.
The answer is not to buy more paint but to start with a coat of primer: it costs little, evens out absorption and saves real paint. On a new or skimmed wall it nearly always pays for itself.
How many coats are really needed
Two, almost always. One shows: the roller laps stay visible, and the colour underneath comes through in raking light even when it looks covered. Three are needed when going from a dark colour to a light one, when covering a damp or smoke stain, or when using a red, a yellow or an orange, which are the least opaque pigments of all.
On a stain there is something paint does not solve: if the cause is still active, the mark comes back through any number of coats. First dry it and seal it, then paint.
Openings, and when not to subtract them
Subtracting doors and windows is right when they are large. But if the openings are small and numerous, taking them all out means buying too little, because around every opening there is an edge to be cut in with a brush, and a brush uses more than a roller. Site practice is to subtract only surfaces above a square metre and ignore the rest.
The ceiling, on the other hand, has to be added if you are painting it, and it is not included in perimeter times height: it is the floor area, and it is the item most often forgotten in the sum.
Buy whole tins, and from the same batch
Paint is sold in fixed sizes and has to be rounded up, but there is one more reason not to cut it fine: two tins of the same colour bought at different times may come from different tinting batches, and on a large wall the difference shows in raking light. If more than one tin is needed, mix them all together in a bucket before starting.
Leftovers should be sealed well and kept away from frost: they will be needed for touch-ups, which are the only way to fix a chair mark without repainting the whole wall.
Nearby tools
The surface to cover is worked out by Room area and volume. For the floor there is How many tiles you need, for decorated walls How much wallpaper you need and for the light in the finished room How many lumens you need.