How the arithmetic works
Paint coverage is an area problem, and the only genuinely reliable way to do it is perimeter × height. Measuring each wall separately invites arithmetic errors and takes four times as long; a tape run around the skirting gives you the same number in one pass.
From that gross area you subtract openings. A standard internal door is about 1.9 m² and a typical domestic window about 1.4 m², and those are the figures used here. They are approximations — if you have patio doors or a bay window, measure them, because a large opening changes the answer materially.
The result is then multiplied by the number of coats and divided by the spread rate. That last number is the one people get wrong most often, and it is the only figure in the calculation you cannot derive: it has to come off the tin.
Spread rate is not a constant
Manufacturers quote coverage per litre under ideal conditions — a smooth, sealed, previously painted surface. Real walls rarely are. Bare plaster drinks the first coat and can halve the effective coverage. Textured or previously distempered surfaces do the same.
The practical rule: if you are painting over existing emulsion in good condition, trust the tin. If you are painting bare plaster, assume the mist coat covers at roughly half the quoted rate and buy accordingly. If you are going from a dark colour to a light one, plan on three coats rather than two and let the calculator do the multiplication.
| Surface | Effective coverage | Coats to plan for |
|---|---|---|
| Existing emulsion, good condition | As quoted on the tin | 2 |
| New plaster | Roughly half on the mist coat | 1 mist + 2 |
| Dark colour going lighter | As quoted | 3 |
| Textured or artex | 20–30% below quoted | 2 |
| Bare wood or MDF | Well below quoted | Primer + 2 |
What people get wrong
The most expensive mistake is buying in the wrong tin size. Paint is markedly cheaper per litre in 5 L tins than in 2.5 L, so a job needing 5.5 L is usually better served by a 5 L plus a 2.5 L than by three 2.5 L tins. The calculator tells you the volume; the tin arithmetic is worth doing at the shelf.
The second is forgetting the ceiling. This calculator covers walls only, because ceilings are a different area calculation — length × width of the room, not perimeter × height. Run it twice if you are doing both.
The third is buying exactly enough. Batch variation between tins of the same colour is real, and running out mid-wall means a visible join. Buy the round-up, not the exact figure.
Common questions
How much paint for a 12 × 12 room?
A 12 × 12 ft room has a 48 ft perimeter. At an 8 ft ceiling that is 384 ft² gross, roughly 350 ft² after a door and a window, and about 700 ft² over two coats — a little under 2 US gallons at a typical 350 ft² per gallon spread rate.
Does the calculator include the ceiling?
No. Ceiling area is length × width, not perimeter × height, so it is a separate sum. Run the calculator a second time using the ceiling dimensions if you need both.
Should I buy extra?
Yes, round up. Touch-ups years later are far easier with a part tin of the exact batch than with a fresh tin that has drifted slightly in colour.