This calculator turns a room's dimensions into litres of paint, whole tins and a cost figure. Enter the floor length and width, the ceiling height, how many doors and windows break up the walls, the number of coats, and the coverage printed on the tin. It returns the wall area, the openings deduction, the paintable area, the litres required, the can count and the cost.
The decision it supports is simple: how many tins go in the trolley. Paint is sold in fixed can sizes, so the answer is never “6.62 litres” but “two 5-litre tins”. Under-buy and you make a second trip mid-job, and a second batch of tinted paint will not match the first exactly.
Everything is computed in the selected system: metres with m²/L and litres, or feet with ft²/gal and gallons. The two sets of defaults are independent figures, not conversions of each other.
wall area = 2 × (length + width) × height — the four walls unwrapped.ceiling area = length × width, added only when the ceiling box is ticked.openings = doors × 1.9 m² + windows × 1.4 m². These are fixed allowances, roughly a 0.8 × 2.4 m door leaf with its frame and a 1.0 × 1.4 m window, not measurements of your actual joinery.paintable area = wall area + ceiling − openings, floored at zero.litres = paintable area × coats ÷ coverage per litre. Coverage is a one-coat figure, which is why coats multiplies the area and not the volume.cans = litres ÷ can size, rounded up.That last line is the only slack in the model. No waste percentage is applied anywhere: the coverage figure is taken at face value, and the round-up to whole cans is the entire contingency.
A living room 5.0 m by 4.0 m with a 2.5 m ceiling, two doors, two windows, two coats, a tin stating 10 m²/L, and 5 L cans at £32.
| Step | Calculation | Result |
|---|---|---|
| Wall area | 2 × (5.0 + 4.0) × 2.5 | 45.0 m² |
| Doors and windows | (2 × 1.9) + (2 × 1.4) | − 6.6 m² |
| Area to paint | 45.0 − 6.6 | 38.4 m² |
| Paint needed | 38.4 × 2 ÷ 10 | 7.68 L |
| Cans of 5 L | 7.68 ÷ 5, rounded up | 2 |
| Estimated cost | 2 × £32 | £64 |
Can size moves the answer more than the quantity does: those 7.68 litres are one 10 L can, two 5 L cans, four 2.5 L cans or eight 1 L cans.
Ten square metres per litre is a best case, measured on a smooth, sealed, previously painted surface. Bare or new plaster drinks the first coat and can halve the effective coverage; on fresh plaster, set coverage to about 6 m²/L and coats to 3 — a thinned mist coat plus two finish coats.
The openings allowance is typical, not measured. A glazed run of French doors has far more opening than the tool deducts, so that room is over-estimated, while a windowless box is under-deducted. For unusual joinery, enter zero door and window counts and subtract the real area yourself.
Dark to light needs an extra coat. There is no field for “the wall is currently navy”, so you supply that knowledge by raising the coat count or budgeting a tinted primer as an extra coat.
Drying time sets the schedule, not quantity. Water-based emulsion is typically recoatable in 2–4 hours, so both coats go on in a day; oil-based gloss and eggshell typically want 16–24 hours between coats, making a two-coat room a two-day job. Buy enough to finish a wall in one session, and do not split a room across two tins, because a seam can show where two batches meet.
Yes, and the input is labelled that way. Ten square metres per litre means one litre covers ten square metres once, so two coats need twice the volume.
Only if you are painting it. It adds length × width — in the example above another 20 m² per coat, about 4 litres over two coats.
Because paint does not go over them, and in a small room those openings can be a seventh of the wall area. Ignoring the deduction is the usual reason people arrive home with one tin too many.
Only if it covers at the same rate as the topcoat. The tool uses one coverage figure for every coat, so for a primer at 6 m²/L under a finish at 12 m²/L, run the calculator twice and add the tins together.
Measure the perimeter, split it into a length and a width that add up to half of it, and enter those. Alcoves will not be exact, but the openings deduction and the can round-up absorb much of the error.