How Much Paint a Room Takes, and Why the Third Coat Is the Expensive One

Paint quantity is one of the few decorating numbers that is genuinely calculable, and getting it wrong in either direction is annoying: too little means a second trip and a possible batch mismatch, too much means tins in a cupboard for a decade.

Wall area first

For a room with a 18 m perimeter and a 2.4 m ceiling, the wall area is 43.2 m² before any openings come off. Subtracting a door and two windows leaves 38.4 m² — 4.8 m² less, or 11.1% of the total.

Openings	Net wall area	Paint (2 coats)	In gallons
0 doors, 0 windows	43.2 m²	8.6 L	2.3 gal
1 door, 1 window	39.9 m²	8 L	2.1 gal
1 door, 2 windows	38.4 m²	7.7 L	2 gal
2 doors, 3 windows	35.1 m²	7 L	1.9 gal

Skipping the subtraction is the commonest source of over-ordering. On this room it is roughly 1 litres of paint you did not need.

Coats multiply everything

Two coats on 38.4 m² is 7.7 litres. Three is 11.5 — 3.8 litres more, which is 50% on top for one extra pass.

That is the number to weigh before deciding a third coat is a nice idea. Sometimes it is unavoidable — a strong colour change, a patchy surface, a pale colour over a dark one — and knowing what it costs lets you plan for it rather than discovering it two-thirds of the way through.

What drives coverage

The calculation assumes 10 m² per litre, which is a reasonable average. Real coverage moves with:

Surface texture. Anything textured has more actual surface than its measured area and drinks accordingly.

Porosity. New plaster, filler and bare wood absorb far more on the first coat, which is what primer and mist coats are for.

Colour change. Going pale over dark needs more coats, and the paint itself does not cover further because you want it to.

Application. A roller lays down more than a brush; spraying more again, with more overspray.

Buy in the tin sizes that exist

7.7 litres is not a purchasable quantity. Paint comes in fixed sizes, so the real order rounds up — and as with anything sold by the container, the rounding on a small job can be a large proportion of it.

Buying slightly over is the right instinct anyway: touch-ups years later from the original tin match, and a fresh tin of the same colour frequently does not.

What the number leaves out

Ceiling, woodwork and any primer are all separate calculations. The figure above is walls, in the wall colour, and a full room repaint is usually three different products in three different quantities.

Also left out: the ceiling area entirely. If you are painting it, the square footage calculator gives you the floor area, which is the same number.

Working through your own room

Measure the perimeter rather than each wall — it is one tape run and it is what the paint calculator asks for. Count doors and windows honestly. Decide the coats before ordering rather than after the first one looks thin.

And if the room is unusually tall, note that wall area scales with ceiling height directly: the same 18 m perimeter at 3.2 m instead of 2.4 m is 57.6 m², nearly 33.3% more wall for the same floor.

Why a mist coat is not a wasted coat

On new plaster, the first coat of ordinary emulsion sits on the surface and peels later. A thinned mist coat soaks in and gives the following coats something to bond to, which is why it is a separate step rather than an optional one.

It also uses less paint per square metre than a full coat, so budgeting a mist coat as a whole coat over-orders. Treat it as roughly half.

Order of work, which changes the quantities

Ceiling first, because the splatter lands on walls you have not painted yet.

Then walls.

Woodwork last, and in a different product — trim paint covers differently and is calculated by linear metre rather than area, so it is not in this figure at all.

Doing it in another order does not waste paint directly; it wastes the time spent fixing what landed on the finished surface.

What to keep afterwards

Keep at least half a litre of the wall colour, labelled with the room and the date. Touch-ups from the original tin are invisible; touch-ups from a freshly mixed tin of nominally the same colour frequently are not, because mixing is done in batches.

Store it somewhere that does not freeze. A tin left in an unheated garage over winter is usually ruined.

Running your own room

One tape measure round the perimeter, the ceiling height, and an honest count of doors and windows. The paint calculator does the rest, and the only judgement call is the coat count — which, as the table shows, is the input that moves the answer most.

Two coats or three, decided honestly

Two is enough for a similar colour over a sound, previously painted surface in good condition. This is the majority of repaints.

Three is realistic for a pale colour over a dark one, a strong or highly saturated colour, new plaster after its mist coat, or any wall that has been patched.

The cost of guessing low is a half-finished room and a second trip; the cost of guessing high is 3.8 litres in a cupboard. Those are not symmetrical inconveniences, which is an argument for ordering as though three were likely and being pleased if two does it.

One number the calculator will not give you

How long it takes. Paint quantity is arithmetic; the job is not. A room this size is comfortably a weekend once preparation, cutting in, drying time between coats and putting the room back together are counted — and preparation is most of it.

Budget the time the way you budget the paint: generously, and for three coats rather than two.