The calculation, in one line
Paint quantity is not complicated arithmetic. It is:
volume = (paintable area × number of coats) ÷ spreading rate
The paintable area is the four walls minus the doors and windows. The spreading rate is printed on the tin. The number of coats is a judgement about opacity. Everything that goes wrong with a paint estimate goes wrong in one of those three inputs, not in the division.
For a room 4 metres by 4 with a 2.4 metre ceiling: the walls are 2 × (4 + 4) × 2.4 = 38.4 square metres. Take off a door at 1.51 and a window at 1.5, and you have 35.4 square metres to cover. Two coats is 70.8 square metres of coverage, and at 12 square metres per litre that is 5.9 litres — so a 5 litre tin and a 1 litre tin, or a 10 litre tin if you would rather have the surplus.
The spreading rate is the number that decides everything
Every other input is a measurement. This one is a claim, and it is the input most worth being sceptical about.
Manufacturers declare a spreading rate — commonly 10 to 14 square metres per litre for interior emulsion, or 350 to 400 square feet per gallon in the United States. That figure is measured under favourable conditions: a smooth, sound, sealed surface that has been painted before, applied by an experienced hand at the intended film thickness.
Your wall is probably not that. The things that push real usage above the printed figure:
Absorbency. New plaster and fresh plasterboard drink paint. A first coat on bare plaster can consume half again what the tin claims, which is why a thinned mist coat is the standard first step rather than a full-strength one.
Texture. Any relief on the surface increases its true area well beyond its measured area. Textured wallpaper, artex, brick and rough render can take twice the smooth-surface figure.
Colour change. Coverage figures assume you are achieving opacity in the stated number of coats. Going from a dark colour to a pale one, or covering a saturated red or yellow, may not reach opacity in two coats at any spreading rate.
Application method. Rolling puts down more paint than brushing, and spraying more still once overspray is counted.
Treat the calculated figure as a floor. It is the least you could need if everything goes well.
How many coats you actually need
The number of coats is the input people get wrong most often, and it multiplies straight through the result — getting it wrong by one changes the answer by fifty per cent on a two-coat job.
One coat works only when you are repainting the same colour, on a sound, previously painted, non-absorbent surface. It is a refresh, not a decoration job.
Two coats is the normal answer and what you should assume for almost any colour change on a prepared surface.
Three coats, or two over a primer, for the hard cases: pale over dark, anything over a strong red, orange or yellow, bare wood or metal, and stained or patched surfaces where a stain-blocking primer is doing the covering rather than the topcoat.
There is a false economy worth naming here. Budget trade emulsions typically carry less pigment and therefore less opacity, so they need an extra coat to reach the same finish. Once you count the additional tin and the extra day, the cheaper paint frequently costs more than the premium one and takes longer to apply.
Doors and windows matter less than you think
Deducting openings feels significant and mostly is not, which is worth knowing so you spend your attention on the inputs that do move the answer.
A standard UK internal door is 1981 by 762 millimetres — about 1.51 square metres. A US interior door at 80 by 30 inches works out at nearly the same, around 1.55. A mid-size window is roughly 1.5 square metres, though windows vary far more than doors do.
On the 38 square metre room above, deducting one of each removes 3 square metres — under eight per cent, and around half a litre across two coats. It is worth doing, and it will rarely change which tins you buy.
Two cases where it does matter: rooms with a lot of glass, such as a conservatory or a room with patio doors, where openings can be a quarter of the wall area; and large jobs, where eight per cent of a big number is a tin.
Note that this deduction is for the wall area behind the opening. It says nothing about painting the door itself or the window frame, which is usually a different product — a gloss, satinwood or eggshell rather than a wall emulsion — and needs its own estimate.
Ceilings are a separate calculation
The tool keeps the ceiling optional and reports it separately, and the reason is practical rather than tidy.
Ceilings are usually painted with a dedicated ceiling paint: a flat, matt, low-sheen product that hides unevenness where a wall emulsion would show every roller lap under a light. If your ceiling paint is a different product from your wall paint, adding the two areas together gives you a total you cannot buy as one tin, and you would be short on both.
Include the ceiling in the figure only when you are genuinely using the same tin on both. If you are not, run the calculation twice — once for the walls at the wall paint's spreading rate, once for the ceiling area at the ceiling paint's.
A ceiling is also nearly always more absorbent than expected, and white-on-white is deceptively hard to cover evenly. Two coats is the realistic assumption there too.
Buy it all at once, from one batch
This is the piece of advice most likely to save you a visible mistake, and no calculator can enforce it.
Tinted paint is mixed in batches, and two tins of nominally the same colour from different batches can differ slightly. On a small area you will never notice. Across a single large wall, a join where one batch ended and another began can be plainly visible in raking light, and there is no fix short of repainting the whole wall.
So: work out what you need, add a margin, and buy it in one go. If the job is large enough to need several tins of the same colour, professional practice is to "box" them — pour them all into one larger container and mix, so that any batch difference is distributed evenly and invisibly rather than concentrated at a boundary.
Keep the leftover, labelled with the room and the date. A matched tin for touching up a scuff two years later is worth more than the shelf space it costs.
Working in US units
The tool handles both systems, and the conversion holds one trap.
Enter dimensions in feet, and coverage in square feet per US gallon. Typical US figures of 350 to 400 square feet per gallon correspond to about 8.6 to 9.8 square metres per litre — noticeably more conservative than the 10 to 14 commonly printed on UK tins, which reflects a difference in how the figure is declared rather than a difference in the paint.
The trap is the gallon. A US gallon is 3.785 litres; an imperial gallon is 4.546. They are not interchangeable, and a coverage figure taken from a British tin cannot be used with US gallons without converting first. If you are reading a datasheet, check which gallon it means before you divide by it.
What this calculation does not cover
Primer and undercoat. These are separate products with their own spreading rates, usually poorer than a topcoat's. If your surface needs priming, calculate it as its own single-coat job.
Woodwork and metalwork. Skirting, architrave, doors, frames and radiators take a different product and are measured in linear metres and unit counts, not wall area.
Exterior work. Masonry paint on render or brick can take two to three times what an interior wall does, and exterior wood needs a system rather than a coat.
Wastage. Paint left in the tray, on the roller, and in the tin you could not quite get out is real and is not in the figure above. It is another reason the calculated volume is a floor.