Paint Calculator

Measure each wall across and add the widths together — say four walls coming to 14 metres in total, 2.4 metres floor to ceiling, with a door and a window making about 3.5 m² you are not painting. Smooth plaster, two coats. The calculator returns 30.1 m² of net wall, 6 litres of paint, and 2 tins at 5 litres.

Adding the wall widths is deliberate rather than a shortcut. A perimeter formula assumes the room is a rectangle, and rooms have chimney breasts, alcoves, bays and returns. Measuring each wall and adding gives the same 14 metres for a plain room and the right answer for every room that is not.

One coat is 3.01 litres; two coats is 6.02; tins round up to 2 because paint is sold by the tin. The area is the wall, not the wall times the coats — the coats are in the litres, which is where they belong.

Switch Surface Type to Ceiling and the second field renames itself from Wall Height to Ceiling Length, because a ceiling has no height — it has the other side of the room. A 4 by 3 metre ceiling is 12 m², two coats, 2.7 litres, one tin.

The coverage rate drops to about 9 m² per litre where a wall gets 10. Ceiling paint is flatter, more absorbent stock, and a lot of ceilings carry some texture whether anyone chose it or not. Do the ceiling as its own run rather than adding its area to the walls: it is a different product in a different tin, and a combined figure would be telling you to buy something nobody sells.

Twelve metres of wall at 2.4 high is 28.8 m² — almost exactly the same as the plastered room above. Set the texture to bare brick or block and it needs 10.5 litres and 3 tins, against 6 litres and 2 tins for the smooth room.

Nearly double the paint for the same area, and it is not waste. Coverage falls from 10 m² per litre to 5.5 because the film has to bridge open mortar joints and the pores of the block rather than sit on a flat surface. This is the single biggest number on the page, and it is the one most often left at its default.

Ignoring texture. It is the single biggest thing on this page. Smooth plaster takes about 10 m² per litre; light texture 8.5; heavy texture or Artex around 7; and bare brick or block barely 5.5, because the film has to bridge the profile rather than sit on it. That is nearly double the paint for the same wall.

The second one is treating a ceiling as a wall. Ceiling paint is flatter, more absorbent stock and covers about 10% less for the same area — and it is a different product, so you cannot pour one into the other. Do the ceiling as a separate run rather than adding its area to the walls; a combined tin count would be telling you to buy something that does not exist.

Litres, or tins of a size you have chosen. The Tin Size field is there because tins vary by brand and by product — 2.5, 5 and 10 litres are all common — and the rounding is what actually decides your bill.

Buy the whole job in one go where you can. Paint is batch-mixed and colour varies slightly between batches, which shows on a wall that is cut in on Saturday and rolled on Sunday from a different tin. If a job is close to a tin boundary, going up is usually the cheaper mistake.

The Paint Needed row states the coverage rate it used and the texture it came from, so the assumption is on screen rather than buried. Manufacturers print their own spread rate on the tin; if theirs differs, theirs wins — it is their paint.

Primer is quoted only when you ask for it. Over sound existing paintwork you do not need it, and a calculator that adds a primer coat to every job is quietly inflating every estimate it produces. Bare or patchy surfaces are a different matter, and that is what the dropdown is for.