Most people buy paint the same way: pace out the room, guess, then add a bucket for safety. The guess is usually wrong in the expensive direction, and the spare bucket sits in a store room until it is past its shelf life.
The arithmetic takes about two minutes and needs one number from the manufacturer.
The formula
Wall area divided by spread rate, multiplied by the number of coats, equals litres.
Wall area is the part people get wrong, so take it slowly. Measure the perimeter of the room, which is every wall length added together, then multiply by the ceiling height. A room 4 metres by 5 metres has a perimeter of 18 metres. At 3 metres high that is 54 square metres of wall.
Then subtract openings. A standard door is about 1.8 square metres and an average window about 1.5. Two doors and two windows takes roughly 6.6 square metres off, leaving about 47.
Do not skip the subtraction on a small room. On a bedroom with a door and two windows you can be overestimating by 15 per cent, which is most of a 4 litre tin.
Where spread rate comes from
Spread rate is how many square metres one litre covers in a single coat. It is the number that decides everything, and it varies more than people expect.
| Product | Spread rate per litre | 20 litres covers |
|---|---|---|
| Acrylic Emulsion | 6 to 8 sqm | 120 to 160 sqm |
| Ruby Matt | 10 to 12 sqm | 200 to 240 sqm |
| Quality Acrylic Satin | 12 to 14 sqm | 240 to 280 sqm |
| Ruby Satin | 14 to 16 sqm | 280 to 320 sqm |
| Maxshield | 16 to 18 sqm | 320 to 360 sqm |
| Weather King Satin | 16 to 18 sqm | 320 to 360 sqm |
| Sungard | 18 to 20 sqm | 360 to 400 sqm |
Always calculate on the lower number. The upper figure assumes a smooth, sealed, previously painted wall and an experienced hand with a roller. Fresh plaster, rough render or a heavy colour change will pull you toward the bottom of the range or below it.
Worked example
The 47 square metre bedroom from earlier, two coats, using Ruby Matt at 10 square metres per litre.
47 divided by 10 is 4.7 litres per coat. Two coats is 9.4 litres. So two 4 litre tins will not do it and you need either three 4 litre tins with some left over, or a 20 litre bucket with a lot left over.
This is the moment to check whether other rooms are getting the same colour. Three bedrooms in the same shade turns 9.4 litres into 28, and now the 20 litre bucket is the obvious buy rather than the wasteful one.
Batch your rooms by colour before you decide pack sizes. It is the single biggest saving available on a repaint, and it costs nothing.
The things that quietly double your paint
New plaster drinks paint. Bare, unsealed plaster or screed can absorb half again as much on the first coat, because it is porous and the binder soaks in instead of sitting on top. Seal it with a primer first. A primer coat is cheaper than a wasted coat of finish paint, and it makes the topcoat behave the way the spread rate says it should.
Dark to light is a three coat job. Going from a deep colour to white or cream almost never covers in two, whatever the tin says. Budget three coats or use a primer to knock the old colour back first.
Texture eats litres. Rough render, textured coatings and blockwork have more surface area than they appear to. Add 20 to 30 per cent to any calculation on a textured wall.
Spraying uses more than rolling. It is faster and gives a better finish on large exterior runs, but overspray and atomisation losses mean you should add roughly a third to your volume.
Ceilings are usually forgotten entirely. A 4 by 5 metre room has 20 square metres of ceiling, which is a meaningful share of the job.
Rough figures for a whole house
If you want a starting point before measuring, these are the internal wall areas we see most often. Treat them as a sanity check, not a quote.
- One bedroom flat, roughly 120 to 150 square metres of wall
- Two bedroom flat, roughly 200 to 250
- Three bedroom flat, roughly 300 to 350
- Four bedroom duplex, roughly 500 to 650
Take a three bedroom flat at 320 square metres, two coats, so 640 square metres of coverage. With Ruby Matt at 10 square metres per litre that is 64 litres, which means four 20 litre buckets. With Acrylic Emulsion at 6 it is 107 litres, so six buckets.
Exteriors are harder to generalise because they depend on the building footprint and how many storeys you are covering, so measure those properly rather than working from a table.
Buy slightly over, but only slightly
Add about 10 per cent for touch ups and for the paint that stays in the tray and on the roller. Do not add a whole spare bucket.
Two reasons. Paint has a 12 month shelf life, so a spare bucket is not a long term asset. And tinted colours are mixed per order, so an exact match later is more reliable if you come back to us with the same shade reference than if you rely on a part-used tin that has thickened.
If you are choosing a tinted shade, note the reference down. Our colour library lists over 2,400 notations with their values, so you can record exactly what you used and reorder it in two years without guessing.
Or let us do it
Every product page on this site has a paint calculator on it. Enter your wall area and it works out the litres for that specific product at its real spread rate, so you are not converting between numbers yourself.
If the job is bigger than a couple of rooms, send us the areas and the surfaces instead. Tell us what the walls are, whether they have been painted before, and roughly how big, and we will come back with the product, the litre count and the cost. Browse the range or get in touch.