The two things that make paint calculators wrong
Working out paint is not difficult arithmetic. It is wall area, less the openings, times the number of coats, divided by the coverage. What goes wrong is that most calculators quietly skip two of those four steps.
Coverage on the tin is per coat
This is the big one. Every tin states its coverage, usually 12 to 14 square metres a litre for emulsion, and that figure is for one coat. Emulsion almost always needs two.
So a 30 square metre room is 60 square metres of covering, not 30. A calculator that divides the room area by the tin figure sends you home with half the paint you need, and you find out halfway through the second coat on a Sunday afternoon.
Doors and windows are not a rounding error
An internal door is about 1.5 square metres. A decent window is another 1.5 to 2. In a normal bedroom that is 10 to 20 per cent of the wall gone, and a lounge with patio doors can lose a third.
Two coats of an overestimate that size is a whole 5 litre tin you did not need, sitting in the garage until it goes off.
How much does a room take?
For a rough sense check before you measure anything: a standard 4m by 3m bedroom with a 2.4m ceiling comes to about 33 square metres of wall. Take off a door and a window and you are at roughly 30. Two coats at 12 square metres a litre is 5 litres.
Add the ceiling, which is another 12 square metres, and you want nearer 7. A large lounge at 5m by 4m with two big windows lands around 9 to 10 litres for walls and ceiling in two coats.
Which tins to buy
Paint gets cheaper per litre as the tin gets bigger, so this always suggests the largest tins first. Two 5 litre tins beat four 2.5s on price everywhere that sells paint.
Buy it all in one go, and check the batch numbers match. Colour varies very slightly between mixes, and a top-up tin bought a fortnight later can be visibly different across a big wall in daylight.
New plaster is a different job
Bare plaster is porous and will drink the first coat straight in. Painting it normally gives a patchy finish that peels later.
The trade answer is a mist coat first: ordinary matt emulsion thinned with about 30 per cent water, which soaks in and seals the surface. Then two full coats on top. Budget for three coats in total on new plaster, and expect the mist coat to go further than the tin says because it is thinned.
Dark over light, and light over dark
Going darker usually covers in two. Going lighter over a strong colour often needs three, and sometimes a tinted undercoat is quicker and cheaper than a third coat of expensive colour.
Strong reds and yellows are the worst for this. Their pigments are naturally translucent, and three or four coats is normal rather than a sign you have done something wrong.
Common questions
How much paint do I need for one room?
A typical 4m by 3m room with a 2.4m ceiling has about 33 square metres of wall. Take off a door and a window and you are near 30. Two coats at 12 square metres per litre needs 5 litres. Add the ceiling and it is nearer 7.
Does the coverage on the tin include two coats?
No. Coverage printed on a tin is per coat, every time. Emulsion normally needs two, so the paint needed is double what a single-coat figure suggests. This is the most common reason people run out.
Should I take off doors and windows?
Yes. In a normal room they are 10 to 20 per cent of the wall area, and in a room with patio doors it can be a third. Calculators that ignore them send you home with a tin you did not need.
How much paint for new plaster?
More than the tin says. Bare plaster is porous and drinks the first coat. Use a mist coat first, which is emulsion thinned about 30 per cent with water, then two full coats on top.
Is it better to buy one big tin or several small ones?
One big one, nearly always. Paint is cheaper per litre in larger tins, and colour varies very slightly between mixes, so buying it all together in one batch avoids a visible difference on a large wall.