Paint is sold by the litre and used by the square metre, and the exchange rate — the spreading rate on the tin — is a figure that assumes a smooth, sealed, already-painted surface. Bare plaster is not that. It drinks the first coat almost entirely, which is why the primer coat is a separate quantity rather than one of the two. This takes the surface, subtracts the openings, applies the rate and the number of coats, and then turns the result into cans — because paint is sold whole and the last can is the one that decides whether you have over-ordered.
Surface, coats and can size
Two coats is not twice one coat, but it is close.
Round the room and multiply by the height, before subtracting openings.
Doors and windows, counted at 1.8 m² each.
On the tin. Around 11 for a good emulsion on a sealed surface.
Left in the tin, on the roller and in the tray.
Result
The spreading rate is a best case
The figure on the tin — typically around eleven square metres per litre for a decent emulsion — is measured on a smooth, sealed, already-decorated surface. It is a reasonable number for the second coat of a redecoration and a bad number for the first coat on new plaster, which is porous enough to absorb a large share of the first coat into the surface rather than leaving it on top.
That is the whole argument for treating primer or mist coat as a separate quantity. A mist coat is emulsion thinned with water, and its job is to seal the surface so that the real coats sit on top of it and cover at the rate the tin claims. Without it, the first coat cover badly and the second one drinks too, and the job uses half again as much paint as it should.
Doors, windows and the subtraction
Subtracting the openings is the one piece of arithmetic people argue about, because it is tempting to be precise and not worth being. A door is about 1.8 square metres and a window about the same, and counting them as openings rather than measuring each one is accurate enough: the difference between 1.8 and 1.9 across four openings is four tenths of a square metre, which is under a twentieth of a litre.
What is worth being careful about is the reverse error — not subtracting them at all — and the framing. A door is usually painted with trim paint, not wall paint, and so is the architrave around it; so is the skirting; so is the window frame. If the job includes the woodwork, it is a separate quantity with its own rate, and it is usually sold in a smaller can.
Two 2.5 litre cans are usually better than one 5 litre, because the job that needs 7.3 litres finishes with 2.7 left in a 10 litre can and nothing left in a 5 litre.
Keep the leftover, labelled with the room and the date. Paint matched later from a chart rarely matches, and a touch-up from the original tin always does.
Colour changes cost coats
Going from a light colour to a similar light colour is one coat. Going from light to dark, or dark to light, is two at minimum and often three, because the old colour shows through the first one. Going from white to a deep red or a strong blue is the worst case in the trade, and the answer is not more coats of the top colour but a grey or tinted primer underneath.
The reason is opacity, and it is worth knowing that it varies by pigment. White and pastel emulsions are loaded with titanium dioxide, which covers well. Deep reds, oranges and yellows use organic pigments that are much more transparent, which is why a feature wall in a strong colour routinely needs three coats where the same wall in cream needed one.
| Job | Coats | Primer | Note |
|---|---|---|---|
| Like over like, light | 1 | No | One coat often genuinely suffices |
| Like over like, mid tone | 2 | No | The default redecoration |
| New plaster | 2 | Mist coat | The mist coat is not optional |
| Light over dark | 2 to 3 | Tinted | Grey primer saves a coat |
| Dark over light | 3 | Tinted | Deep pigments are transparent |
| Bare wood or metal | 2 | Yes | Primer and undercoat first |
A worked example
- Surface40 m² of wall, less two openings at 1.8 m², leaves 36.4 m².
- Two coatsAt 11 m² per litre that is 6.62 litres.
- AllowanceTen percent takes it to 7.28 litres.
- CansIn 5 litre cans that is 2 cans, or $90 at $45.
- The primerOne sealing coat is 3.31 litres — a third of a can if you already have paint, a whole one if you do not.
Two cans for a forty square metre room is roughly right for a redecoration in a similar colour. The same room in new plaster, or in a dark colour over a light one, is a three-can job, and the difference is entirely in the coats rather than in the surface.
Getting the surface measured
Measure the perimeter along the floor and multiply by the height, then subtract the openings. It is faster and more reliable than measuring four walls individually, because the perimeter is one tape run and it catches the returns into a bay or a chimney breast automatically if you follow the skirting.
For a chimney breast or a bay, follow the skirting line rather than the wall centre line, because the surface is at the wall face and the two differ by the thickness of the returns. On a stair wall, measure the horizontal run and add a third for the slope rather than trying to measure the hypotenuse.
The surface itself is the other half of the job: the wall area figure handles the measuring properly, and the ceiling area figure does the same overhead — ceilings usually take one coat of a different, flatter paint. If the job is being costed rather than just measured, the paint cost figure puts the paint alongside the labour, which is usually the bigger number.
Do
- Measure the perimeter and subtract the openings, rather than measuring four walls.
- Mist coat new plaster before the real coats.
- Use a tinted primer for a big colour change rather than extra top coats.
- Buy the can size that leaves the least over, and keep what is left.
- Count the woodwork as a separate quantity in its own paint.
Avoid
- Believing the spreading rate on bare plaster.
- Painting new plaster without a mist coat and using half again as much paint.
- Trying to go dark over light in two coats.
- Buying one big can when two small ones leave less over.
- Throwing away the leftover and trying to match the colour later.
Where the numbers stop being true
The spreading rate is the weak figure. It varies by brand, by sheen, by colour and by the tool: a roller uses more than a pad, and spraying uses more than either. It also drops sharply on a textured surface — a heavy blown vinyl or an embossed wallpaper can take twice the paint of a flat wall, because the paint is covering the texture as well as the area.
Nothing here accounts for the woodwork, for the ceiling, or for a feature wall in a different colour. Each is a separate quantity with its own rate and usually its own can, and the honest way to handle a room with two colours is to run the calculation twice, once per colour, with the surface split between them.
And the opening allowance is a flat 1.8 square metres. A patio door or a bay window is much more than that, and a room with a full wall of glazing should have that wall measured and subtracted properly rather than counted as an opening.
How the figures are produced
Openings are counted at 1.8 square metres each and subtracted from the gross wall area. Litres are the net area divided by the spreading rate, times the number of coats. The waste percentage is added, and cans are that figure divided by the can size, rounded up — so the final can is part-full. The primer line repeats the calculation for a single coat, which is what a mist coat or a sealing coat needs.
Questions people ask
How much paint for a 40 square metre room?
About 7.3 litres for two coats at 11 m² per litre — two 5 litre cans, with a couple of litres over.
Do I need a primer?
On new plaster, yes — a thinned mist coat. On a big colour change, a tinted one. On a like-for-like redecoration, no.
Is one coat ever enough?
Yes, on a similar light colour over a sound existing coat. Not on new plaster, not over a patch, and not on a colour change.
Should I subtract the windows?
Yes, at about 1.8 m² each. But count the frames and the sills as a separate quantity in trim paint.
Why did I use more than the tin said?
Usually a porous surface, a textured one, or a colour change. All three make the first coat far less effective.
Can I keep the leftover?
Yes, for a couple of years, sealed and labelled with the room. It is the only reliable source for a touch-up.