The number on the tin assumes a perfect wall
Every tin of emulsion carries a coverage figure, and it is usually somewhere around 10 to 12 square metres per litre. That number is real, but it was measured on a smooth, sealed, non-absorbent surface — the best case the paint will ever meet. Roll the same paint onto bare blockwork and you will get roughly half of it.
The reason is simple once you picture the wall in section. Paint covers a surface by forming a continuous film over it. On smooth plaster the true surface area is barely larger than the area you measured with a tape. On a textured or open-pored surface the real area is much greater, and the film also has to bridge and fill the profile rather than simply sit on top of it. You are not just painting a bigger area, you are pouring paint into it.
This is the single largest source of error in a paint estimate, and it is far bigger than the errors people usually worry about. Rounding your room dimensions to the nearest 100 mm changes the answer by a percent or two. Getting the texture wrong changes it by eighty.
The four texture bands
The calculator on this site applies a multiplier to the base coverage rate, which for walls is 10 square metres per litre per coat.
- Smooth plaster or plasterboard: the full 10 m² per litre. Skimmed plaster, lining paper, sound existing paintwork, or new plasterboard that has been taped and filled.
- Light texture: about 8.5 m² per litre. A slightly orange-peel roller finish, woodchip paper, or a wall that has been patched and rubbed back so the surface is faintly uneven.
- Heavy texture or Artex: about 7 m² per litre. Swirled and stippled ceiling textures, deep relief papers, and rendered surfaces with a visible aggregate.
- Bare brick or block: about 5.5 m² per litre. Fair-faced blockwork, common brick, and unrendered masonry, where the film has to bridge open mortar joints and the pores of the block itself.
Ceilings start from a slightly lower base of 9 square metres per litre before the texture multiplier is applied. That is not superstition. Ceiling paints are usually flat finishes over more absorbent stock, and a lot of ceilings carry some texture whether or not anyone chose it.
Measuring the room
For a whole room, the wall area is the perimeter multiplied by the height.
Wall area = 2 × (length + width) × height
A room 4 metres by 3 metres with a 2.4 metre ceiling has a perimeter of 14 metres, so 14 × 2.4 = 33.6 square metres of wall. From that you subtract the openings. A standard door is close to 2 square metres and an average window around 1.5, so with one of each you are painting about 30 square metres.
That formula only holds for a rectangle, which most rooms are not. The reliable way, and the way the calculator asks for it, is to measure each wall across and add those widths together — 4 + 3 + 4 + 3 gives the same 14 metres, and unlike the formula it still works when there is an alcove, a chimney breast or a fifth wall. Enter that total as the width, and your floor-to-ceiling measurement as the wall height.
Do the ceiling as a second run rather than adding it in. Ceiling paint is a different product from wall paint, so it is a separate tin count — a combined figure would be telling you to buy something that does not exist.
At the smooth rate that is 30 ÷ 10 = 3 litres per coat, or 6 litres for two coats. On bare block the same room is 30 ÷ 5.5 = 5.5 litres per coat, or 11 litres for two. Same room, same tape measure, nearly double the paint — which in practice is the difference between two 5 litre tins and three.
If you are painting the ceiling too, add its area separately rather than lumping it in with the walls, because it uses its own coverage rate. The 4 by 3 metre ceiling above is 12 square metres, which at 9 square metres per litre is about 1.3 litres per coat.
What the coverage figure does not include
- The first coat on bare stock: a porous surface takes noticeably more on the first coat than the second, because it is still absorbing. Estimating both coats at the same rate quietly understates the job. A primer or a thinned mist coat is the cheaper way to deal with this.
- Colour changes: covering a dark colour with a pale one is a question of opacity, not coverage. It may take three coats where two would have done, and no coverage figure will warn you.
- Cutting in and waste: brushwork at edges, what stays in the tray, and what dries in the roller. A 10 percent allowance covers it on a normal room.
- Application method: spraying loses material to overspray, and a long-pile roller loaded for a rough wall carries more paint than a short one.
The practical rule
Measure the room honestly, subtract the doors and windows, pick the texture band that matches what you are actually looking at rather than the one you would prefer, and multiply by the number of coats. Then round up to whole tins — and if you are within a few hundred millilitres of the next tin size, buy it. A part tin left over is cheap. Stopping halfway up a wall to fetch another one leaves a visible edge that no amount of care will blend out.