How much paint do I need is the question every repaint starts with, and getting it wrong costs money in both directions. Buy short and you make a second trip mid-job. Buy long and you own three half tins you will never open again.
The maths is simple. Area, divided by coverage, multiplied by coats, plus a wastage allowance.
Step one: how much paint do I need per litre of coverage
Coverage is the number that changes most, and it is the one people guess at. Published South African technical data sheets give these practical rates per coat.
- Interior PVA or acrylic on smooth, previously painted plaster: about 8 to 10 m2 per litre, and up to 10 to 12 m2 on a well-sealed smooth wall
- Exterior wall paint, or rough and bagged plaster: about 6 to 8 m2 per litre
- Plaster primer or sealer: about 6 to 10 m2 per litre
- Textured coatings: roughly 25 per cent less than the smooth equivalent
New, bare plaster drinks the first coat. Work on the lower end of every range for a surface that has never been painted, because the wall absorbs the paint rather than holding it as a film.
Step two: measure the walls properly
You want net paintable area, not floor area. The formula painters use is the room perimeter multiplied by the wall height.
- Add the length and the width of the room, then double it. That is the perimeter.
- Multiply the perimeter by the wall height. That is your gross wall area.
- Deduct the openings. Standard practice is 2 m2 per door and 1.5 m2 per window.
- Measure the ceiling separately if you are painting it, using length times width.
Do not deduct skirting, cornice or door frames from the wall figure. Those are trim, they are painted in a different product, and they are usually quoted separately, as our trim painting cost guide explains.
Step three: coats and wastage
Two coats is the standard specification for a repaint. Plan for three in two situations: a strong colour change, particularly light over dark, and bare or repaired plaster where a sealer coat comes first.
Then add roughly 10 per cent. That covers touch-ups, the paint left in the roller and tray, and patches that soak up more than the rest of the wall. Rounding up to the next sensible tin size usually handles it.
Materials are only part of the budget. Labour is the larger half of a professional job, and the house painting cost Cape Town guide shows how the two split out per square metre.
A worked example for a standard bedroom
Take a 4 m by 4 m bedroom with 2.7 m walls, one door and one window.
- Perimeter: (4 + 4) x 2 = 16 m
- Gross wall area: 16 x 2.7 = 43.2 m2
- Deduct openings: one door at 2 m2 and one window at 1.5 m2 = 3.5 m2
- Net wall area: 39.7 m2
- Two coats: 39.7 x 2 = 79.4 m2 of coverage needed
- At 9 m2 per litre: 79.4 divided by 9 = 8.8 litres
- Plus 10 per cent wastage: about 9.7 litres, so buy a 5 L tin plus a second 5 L tin
The ceiling in that room adds 16 m2, which at two coats is another 3.5 to 4 litres of ceiling paint.
Tin sizes and buying sensibly
South African paint is sold mostly in 1 L, 5 L and 20 L tins, and the price per litre drops sharply as the tin gets bigger.
- A 20 L tin of interior paint covers roughly 160 to 200 m2 per coat, so 80 to 100 m2 at two coats
- A 20 L tin of exterior paint covers roughly 120 to 160 m2 per coat
- Buy one batch at a time for a large area, because tinted colours can vary slightly between batches
If your calculation lands just over a tin size, it is usually cheaper to go up rather than add a 1 L tin later. Leftover paint is also genuinely useful for touch-ups after the job. Paint performance standards in South Africa are set out by the South African Bureau of Standards.
When the calculation stops being the hard part
Estimating litres is easy. Preparation is what decides whether the paint lasts five years or two. Cracks need filling, glossy surfaces need keying, bare plaster needs a sealer, and none of that shows up in a coverage sum.
If you would rather not measure, buy and prep it yourself, our painting team quotes per square metre so the scope is clear before anyone starts. For a sense of the rates, the painters near me cost guide sets out the current per-square-metre bands. Send us your room sizes and we will do the measure and the quote at no charge.
Frequently asked questions
How much paint do I need for a room?
Multiply the room perimeter by the wall height, deduct 2 m2 per door and 1.5 m2 per window, then multiply by the number of coats and divide by the coverage rate. A 4 m by 4 m bedroom with 2.7 m walls needs roughly 8.8 litres for two coats at 9 m2 per litre, or about 9.7 litres with wastage.
How many square metres does a litre of paint cover?
Interior PVA and acrylic cover about 8 to 10 m2 per litre per coat on smooth previously painted plaster, and up to 10 to 12 m2 on a well-sealed smooth wall. Exterior paint and rough or bagged plaster drop to about 6 to 8 m2. Bare new plaster absorbs the first coat, so use the low end.
How many square metres does a 20 litre tin cover?
Around 160 to 200 m2 per coat for interior paint, which is 80 to 100 m2 finished at two coats. Exterior paint covers less, roughly 120 to 160 m2 per coat. Larger tins are considerably cheaper per litre, so if your calculation is close to a tin size it usually pays to buy up rather than top up later.
Do I subtract windows and doors when calculating paint?
Yes. Standard practice in South Africa is to deduct 2 m2 for each door and 1.5 m2 for each average window from the gross wall area. Do not deduct skirting, cornice or frames, because those are painted in a separate enamel or trim product and are normally measured and quoted on their own.
How many coats of paint do I need?
Two coats is standard for a repaint in a similar colour. Allow three when you are going light over dark, when covering a strong colour, or when the surface is bare or newly repaired plaster that needs a sealer coat first. Coastal exposure and dark colours on exteriors also commonly justify a third coat.