What this calculator works out
Painting a room comes down to one area, one coverage rate and a shopping decision. This tool takes a room's length, width and height, or a wall area you already know, works out how much is paintable once doors and windows are taken off, and turns that into litres for the coats you want, a US gallons equivalent, and a tin shopping list for UK sizes (5L, 2.5L, 1L) and US sizes (1 gallon, 1 quart) side by side.
This is a walls-and-ceiling calculator, not a materials tool for the room itself: the square footage calculator handles the floor, and the tile calculator handles a bathroom or kitchen.
How the maths works
Wall area starts from the room's perimeter, the distance around all four walls, multiplied by the height. A 4m by 3.5m room has a 15m perimeter (2 × (4 + 3.5)), which at 2.4m high is 36 m² of wall before anything is taken off. A direct area entry skips this step.
Doors and windows come off next: about 1.9 m² (20 sq ft) per standard door and 1.4 m² (15 sq ft) per standard window, sensible averages rather than a measurement of your own opening. If the deductions ever add up to more than the wall area, an oddly small or window-heavy room, the paintable area is held at zero rather than going negative, and the litres figure follows it down to zero too.
Litres needed is the paintable area, times the coats, divided by the coverage rate, plus the optional 10% contingency. Working the default through: 36 m², minus 3.3 m² for one door and one window, leaves 32.7 m² paintable; at 2 coats and 12 m²/L that is 32.7 × 2 ÷ 12 = 5.45 litres, and with contingency, 5.45 × 1.10 = 5.995 litres, near enough 6.
Coverage rates, and why the UK and US figures do not line up
The rates here, about 12 m²/L for matt and standard emulsion, 13 m²/L for silk and 16 m²/L for gloss, satin and trim, come from Dulux's and Crown Paints' published guidance: Dulux quotes around 13 m²/L for a retail vinyl matt emulsion and 16 m²/L for its wall, trim and ceiling paint, while Crown puts a standard silk emulsion at around 14 m²/L. This calculator sits a little under either brand's maximum figure, since both note real-world coverage runs 10 to 20% below the laboratory number once a textured wall or an older, porous surface is in the mix.
Carry that 12 m²/L rate straight across into US units with no rounding and it becomes just under 489 sq ft per gallon, the figure in the table above, but not what a US brand prints on its own tin. Sherwin-Williams and Behr both quote roughly 350 to 400 sq ft per gallon, closer to 9 or 10 m²/L. Neither side has the maths wrong: UK tins quote closer to a laboratory maximum, while the US brands' headline figure already carries a heavier real-world discount. Buying US-brand paint for a job worked out here is still fine, just check that tin's own figure rather than assume the straight conversion.
A worked example, in UK and US prices
A bedroom, 4m by 3.5m, walls 2.4m high, one door and one window, standard matt emulsion, two coats, contingency on: 36 m² of wall less 3.3 m² for the door and window leaves 32.7 m² paintable, 5.45 litres at 12 m²/L and 5.995 with contingency, call it 6. A greedy fill of UK sizes covers that with one 5L tin and one 1L tin. At around £32 for a 5L tin, buying two outright, the simpler way most people shop rather than mixing sizes, comes to about £64 with paint left over for touch-ups.
The same 5.995 litres is about 1.58 US gallons, covered by a greedy fill of one gallon and three quarts. At around $45 to $55 a gallon, one gallon alone falls short, so the honest US list is two gallons, also what the cost estimate assumes on a US-gallon price basis.
One coat or two, and what a mist coat is
Two coats is standard for any genuine colour change, since a single coat over a different existing colour almost always dries patchy even where it looked solid while wet. One coat can be enough for a same-colour refresh over a wall already in good condition.
New plaster needs a mist coat before either rule applies: the first coat, thinned with roughly 10 to 20% water so it soaks into the porous surface rather than sitting patchily on top, followed by two full-strength coats once it has sealed. A freshly plastered wall typically takes three passes in total, and more paint overall than the same wall in good decorative order.
Ceilings, trim and painting more than one colour
Ceilings are usually painted in their own product, a dedicated ceiling paint or a plain matt, partly for the flatter finish and because a pale ceiling reflects more light into the room. Only switch the ceiling toggle on when the ceiling genuinely shares the walls' paint and colour; otherwise run it as its own short calculation with its own coverage rate.
Trim, skirting and door and window frames usually take a gloss, satin or eggshell finish rather than a wall emulsion, formulated to cover already-painted woodwork cleanly. The gloss/trim option here, about 16 m²/L, is meant for that job rather than painting whole walls in gloss, which is unusual outside a kitchen or bathroom finished for a wipe-clean surface.
Questions people ask
Do I need one coat of paint or two?
Two coats for any real colour change; one coat rarely gives even, opaque coverage once the new colour differs from the old. One coat can be enough for a same-colour refresh over a wall already in good condition. New plaster needs a thinned mist coat first, then two full coats.
Does paint coverage differ on new plaster?
Yes. Bare plaster is porous and absorbs a full-strength coat unevenly, so the first coat is thinned with about 10 to 20% water, a mist coat, to seal the surface rather than sit patchily on top. Two full-strength coats follow it, so new plaster typically takes three passes and more paint overall.
How much should I deduct for doors and windows?
About 1.9 m² (20 sq ft) per standard door and 1.4 m² (15 sq ft) per standard window, sensible averages rather than a measurement of your own opening. For an unusually large window or a pair of French doors, measure it directly and adjust the count or the total area to match.
Why do the UK litre and US gallon coverage figures look so different?
A straight, exact conversion of a UK litre rate into US units gives a much higher sq ft per gallon figure than Sherwin-Williams or Behr print on their own tins, because the big US brands already build a heavier real-world discount into their headline 350 to 400 sq ft figure, while UK tins quote closer to a laboratory maximum. Both are honest, just not the same number converted, which is why this calculator shows both.
Is a 10% contingency always enough?
For a straightforward wall in reasonable condition, yes. Increase it for a heavily textured or previously unpainted surface, or a big jump from a dark colour to a light one, both of which often need closer to three coats' worth of paint in practice.
Should the ceiling be included in the same total?
Only if the ceiling is genuinely going in the same paint and colour as the walls. Ceilings are more often a dedicated ceiling paint or a plain white matt, in which case run it as its own short calculation with its own coverage rate.
Coverage rates are sourced from Dulux and Crown Paints for UK litres, and Sherwin-Williams and Behr for US gallons, and are a planning estimate rather than a guarantee. Actual coverage varies by brand, colour, surface texture and application method; check your own tin for a large order.