Worked examples: two rooms, two coats
Both rooms are taken at 12 m² per litre, which is the figure on a typical emulsion tin, over two coats.
| Room | Wall area | Litres needed | Tins to buy | Spare |
|---|---|---|---|---|
| 5 m × 4.2 m | 44.16 m² | 7.36 L | 1 × 5 L + 1 × 2.5 L (7.50 L) | 0.14 L |
| 4 m × 3 m | 33.6 m² | 5.60 L | 1 × 5 L + 1 × 1 L (6.00 L) | 0.4 L |
Wall area only — doors, windows and any painted ceiling are separate lines.
Coats, not walls
Coverage on the tin is per coat, so the coats are counted separately: the 4 m × 3 m room is 33.6 m² of wall, and two coats at 12 m² per litre is 5.60 litres. Reading the tin as “one coat per litre figure” is how people end up a tin short on the second coat.
Tins, not litres
7.36 litres does not divide into 5 L tins, so the calculator rounds up into the sizes that exist: 1 × 5 L plus 1 × 2.5 L is 7.50 litres, leaving 0.14 L spare. The smaller room needs 5.60 litres, which is a 5 L tin plus a 1 L tin, 6.00 litres with 0.4 L spare. Keeping the spare is deliberate: it is there for touch-ups and for the second coat on an awkward patch.
What the calculator does not include
The room mode covers the walls. A painted ceiling, a radiator, doors and skirting are all extra lines, and a bare-plaster wall will drink a mist coat before the finish coats count for anything.
How the calculator works
- Enter the room length, width and wall height; the wall area follows.
- Coverage per litre and the number of coats give the litres needed.
- The litres are rounded up into real tin sizes.
- Any extra, like a ceiling or a second colour, is added as its own line.
Where to go next
Paint FAQs
How much paint do I need for a 4 m × 3 m room?
5.60 litres over two coats at 12 m² per litre, from 33.6 m² of wall. That rounds to a 5 L tin and a 1 L tin — 6.00 litres.
Do I double the paint for two coats?
The area stays the same, but the tin's coverage applies per coat, so two coats is twice the litres. This room is 33.6 m², which is 5.60 litres at 12 m² per litre over two coats.
Why does the calculator ask for a 2.5 L tin as well as a 5 L?
Because 7.36 litres does not fit 5 L tins. One 5 L plus one 2.5 L is 7.50 litres, which leaves 0.14 L spare for touch-ups.
Should I paint the ceiling with the same paint?
Only if you are painting it — the room mode covers the walls. Ceilings usually take a different, flatter paint, and they are added as their own line.
How many coats of paint do I need?
Two is the usual figure for a colour change over a sound, similar-coloured surface. Bare plaster or a dark colour going over a light wall wants a mist coat or a stain-blocking primer first.
Guidance only. The quantities above are planning estimates from the ProjectCalc calculators, not a quote and not a specification. Check your own measurements and your supplier's current product data before ordering.