Emulsion covers about 11 square metres per litre on a sealed, previously painted wall — so a 4 × 3 m room with 2.4 m walls needs roughly 5.6 litres for two coats once a door and a window are deducted. The figure on the tin assumes ideal conditions, and three ordinary situations push real usage well past it.
Knowing which of the three you are in before you buy is the difference between one trip to the shop and two.
What pushes usage past the tin figure?
Porosity, colour change and application method, roughly in that order of impact.
- New plaster, filler or chalky old paint absorbs far more than the tin promises. That is what a mist coat exists for — thinned emulsion that seals the surface so the finish coats behave normally.
- Going pale over dark is genuinely a three-coat job however good the paint claims to be, and a tinted primer saves a whole coat where the change is dramatic.
- Rollers and sprayers put on more paint per square metre than a brush, and a deep-pile roller more than a short one. Textured surfaces increase the actual area being covered without changing the measured one.
The fourth, less obvious one is a strong colour. Deep reds, yellows and some blues use pigments with poor opacity and routinely need an extra coat regardless of what was underneath.
How many coats does the job actually need?
Two is the working default on a sealed wall in a similar colour. One is only enough for a refresh in the same shade, and three is realistic for a colour change, a new surface, or a strong pigment.
The order matters as much as the count. Ceiling first, then walls, then woodwork — because each stage can splash onto the one below it and not the other way round. Cutting in the ceiling edge before rolling the walls means a wall roller mark on the ceiling is a problem you have already solved.
Does wallpaper work the same way?
No, and the difference is the reason a wallpaper order is calculated in drops rather than area. A standard European roll is 10.05 m long and 0.53 m wide, covering about 5.3 m² — but only if every drop uses the full length, which a patterned paper never does.
| Pattern | Extra to allow |
|---|---|
| Plain or random match | 10% |
| Small repeat, straight match | 10–15% |
| Large repeat, straight match | 15–25% |
| Offset or drop match | 15–25% |
The repeat is what does it. A 64 cm pattern repeat on a 2.4 m wall means each drop is cut to the next whole repeat above 2.4 m, so up to 64 cm of every drop goes in the bin before you have hung anything.
Should you deduct doors and windows for wallpaper?
Generally not, unless they are unusually large. A standard door removes about 1.8 m² of wall, and it also interrupts two drops — the offcuts above and below it are rarely long enough to use elsewhere.
Deducting the area while the wastage stays the same is the most common way a room ends up a roll short. The exception is a wall that is mostly glass, where the deduction is real and worth making.
How do you work out the drops?
Divide the wall run by the roll width to get the number of drops, then work out how many drops come from a roll. A 14 m perimeter divided by 0.53 m is 27 drops; a 10.05 m roll cut into 2.4 m lengths gives four drops with 0.45 m left over, so 27 drops need seven rolls.
Add the repeat and the arithmetic changes. With a 64 cm repeat each drop is cut at 2.56 m rather than 2.4 m, so a roll yields three drops instead of four and the same room needs nine rolls. That is the top of the range in the table above, arriving as two extra rolls rather than as a percentage.
What is the batch number for?
Matching. Wallpaper is printed in runs and colour varies slightly between them, enough to show as a band where two batches meet on a wall. Every roll carries the batch number, and the whole room should come from one.
This is why ordering one roll short and topping up later is a genuine risk rather than a minor inconvenience. Buy the extra roll with the order; an unopened roll is usually returnable and a mismatched wall is not.
Questions people ask
How much paint for a 12 × 12 room? At 2.4 m ceilings that is about 34 m² of wall before deductions, so roughly 6 litres for two coats — plus 2.6 litres for the ceiling if you are doing that too.
Does the calculation include the ceiling? Only if you ask it to. A ceiling is the same area as the floor, and it usually takes a different paint.
Should I buy extra paint? A little, and keep it labelled. Touching in later with the same tin is invisible; touching in with a fresh tin of the same colour often is not.
Can I order one roll short and top up? Only at the risk of a different batch. If the shop can reserve a roll from the same batch, that is the version of this that works.
Measure the net wall area for paint and the perimeter for paper, then let the pattern decide the allowance. The paint calculator works from room dimensions and coats, and the wallpaper calculator converts a wall run into rolls with the repeat included.