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Paint Calculator

Calculators · Added

Enter the room and get a litre or gallon figure you can order against. Every assumption behind the answer is an input you can change — the spread rate, the size of a door, the size of a window, the number of coats — because those are exactly the things that differ between one wall and the next, and a calculator that hides them is guessing on your behalf.

Units
m
m
m

Floor to ceiling.

m²

Blank uses 1.95 m².

m²

Blank uses 1.4 m².

m² per litre. Blank uses 11.

Optional.

How to use the paint calculator

  1. 1Choose metres and litres or feet and gallons; the defaults change to match.
  2. 2Enter the room length, width and floor-to-ceiling height, and tick the ceiling if it is being painted.
  3. 3Set how many doors and windows there are. Leave their sizes blank to use a standard door and window.
  4. 4Pick the number of coats and, if the tin quotes one, the spread rate. Two coats is normal over an existing colour.
  5. 5Choose a can size to get the number of cans to buy, and add a price for a total.

Examples

A standard bedroom, walls only

Input
4 m × 3.5 m × 2.7 m · 1 door · 2 windows · 2 coats
Result
About 6.8 litres — two 5 L cans

40.5 m² of wall less 4.75 m² of openings, doubled for the second coat, at 11 m² per litre.

The same room including the ceiling

Input
As above, ceiling ticked
Result
About 9.4 litres

The ceiling adds 14 m². Ceilings usually take a different paint from the walls, so buy them as separate tins rather than one large one.

A US-quoted job

Input
14 ft × 12 ft × 9 ft · 350 sq ft per gallon · 2 coats
Result
About 2.4 gallons — three 1 gallon cans

The 350 sq ft per gallon on the label is the same claim as 8.6 m² per litre, and it assumes a sealed, previously painted surface.

About the paint calculator

What actually drives the number

Only two things matter arithmetically — the area to be covered and how far a litre goes — and the second is far more variable than the first. Spread rate depends on the surface, the tool, the colour and the person. A roller on smooth plaster gets close to the label. A brush on textured masonry might manage half of it. This is why the calculation is worth doing with your own spread rate as soon as you have one: after the first room, you know it.

The number of coats is the other lever, and it multiplies. Going from two coats to three is a fifty per cent increase in paint, not a small top-up, and it is the change most likely to send you back to the shop. Deciding the colour strategy before buying — primer or extra coat, and which — is worth more than any precision in the measuring.

What the area subtraction contributes is smaller than people expect. A standard door and two windows come to under five square metres, which in a typical bedroom is around twelve per cent of the wall area. Worth subtracting, not worth agonising over. Measuring the room height accurately matters more, because it multiplies against the whole perimeter.

Buying, mixing and keeping the rest

Buy in as few tins as will cover the job. Paint is mixed in batches and two tins of the same colour from different batches can differ enough to see where one ended and the next began, particularly in a mid-tone on a large wall. If several tins are unavoidable, the trade solution is boxing: pour them all into one larger container, stir, and work from that, which averages any difference away instead of drawing a line down your wall.

Leftovers are worth keeping and worth labelling. Write the room and the date on the tin, because a year later the colour name on the lid will not tell you which of the three off-whites in the cupboard went where. Sealed properly and kept off a cold floor, emulsion lasts a few years; if it has separated into a clear layer over a solid one and will not stir back together, it is finished.

One thing this calculator does not attempt is the paint for skirting, architrave, doors and frames. Those are painted with a different product in a much smaller quantity, and are usually estimated per door and per metre of trim rather than by area. A single litre of trim paint covers a surprising amount of woodwork.

Frequently asked questions

Why does the tin claim more coverage than this calculator assumes?
Because the figure on the tin is measured on a smooth, sealed, previously painted test surface, rolled by someone with no interest in the finish looking good. Real walls have texture, absorb differently, and get cut in at the edges with a brush that holds more paint than a roller does. The metric default here is 11 m² per litre against a label claim that is often 12 to 14. If you have painted this particular wall before and know what it drank, your own number beats both.
Do I really need two coats?
Almost always, and three when a light colour is going over a dark one or over anything strongly coloured such as a red or a deep blue. One coat is realistic only when you are refreshing the same colour on a wall in good condition. Where the change is drastic, a tinted primer is cheaper and more reliable than a third top coat, because primer is formulated to block rather than to look good and costs less per litre.
How much does new plaster change the answer?
A great deal. Fresh plaster and new drywall are porous and the first coat soaks in rather than sitting on the surface, so plan for the mist coat — thinned paint, sold or mixed for the purpose — to use around half as much again as this figure suggests for a single coat. Add it as an extra coat here to get closer. Sealing properly first also stops the finish coats drying patchy, which is the visible symptom of skipping it.
Should I subtract the doors and windows at all?
Yes, and this tool does it by default, but it is worth knowing what happens if you do not: on a small room with a large window you would over-order by a can. The reason some tradespeople skip the subtraction is that it builds in a margin, and the reason to keep it is that the margin should be a decision rather than an accident. If you want extra in hand, round the can size up deliberately rather than pretending the window is a wall.

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