Litres, tins and primer for one surface at a time, allowing for texture and coats.
Painting a whole room? Measure each wall, add the widths together and enter the total.
Floor to ceiling.
The doors and windows you are not painting, added up into one figure. Leave it blank if there are none — most ceilings have none.
Only needed on bare or patchy surfaces, not sound paintwork.
Extra to order for roller loss and touch-ups. 5% is typical.
Nothing to work out yet
Enter values for Width, Wall Height, Number of Coats, Tin Size (litres).
One surface at a time: width × height, less the doors and windows you exclude. For a whole room, measure each wall, add the widths together and enter the total — that is the same figure a perimeter would give you, and it still works when the room has an alcove or a chimney breast in it. Walls and ceilings are separate runs rather than one combined answer, because they are different products: ceiling paint is flatter, more absorbent stock covering about 10% less for the same area, and you cannot pour one into the other. Texture then matters more than anything else on the form: smooth plaster gets roughly 10 m² per litre, light texture about 8.5, heavy texture or Artex about 7, and bare brick or block barely 5.5, because the film has to bridge the profile instead of sitting on it. Coats multiply the litres, waste is added to what you buy rather than to the surface you measured, and tins round up because paint is sold by the tin. Primer is quoted only when you say you need it — over sound existing paintwork you do not.
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.
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.