Paint calculator
Work out how much paint to buy for a room, in litres and whole tins. You can see and change every assumption, then copy or print a shopping list. No sign-up.
For previously painted interior walls and ceilings in sound condition. One rectangular room at a time, measured in metres.
How the estimate is worked out
- Wall area = 2 × (length + width) × height.
- Doors and windows you add are taken off the wall area: width × height × how many. Nothing is taken off the ceiling.
- Ceiling area = length × width.
- Paint needed in litres = area × coats ÷ coverage per litre × (1 + allowance ÷ 100).
- Tins = paint needed ÷ tin size, always rounded up to a whole tin. Left over = litres bought − paint needed.
Walls and ceiling are worked out as two separate paints and are never added together. All the arithmetic uses unrounded figures: rounding happens once, at the end, when litres become whole tins.
The starting numbers for coats, coverage, allowance and tin size are examples so that you can finish without a tin in your hand. They are not manufacturer data and not a recommendation. Coverage varies between products, so use the figure printed on the paint you choose.
What this estimate cannot tell you
- It is an estimate. How far paint really goes depends on the surface, the product, the colour change and how it is applied.
- It cannot see your walls. New or bare plaster, damp, mould, stains, flaking paint and glossy surfaces need different preparation or products.
- It does not check that a paint suits the room or the surface. Follow the manufacturer's instructions for the product you choose.
- Old coatings can be hazardous. Paint in older homes can contain lead, and textured wall and ceiling coatings applied before 2000 can contain asbestos. Do not sand or scrape them: get advice first.
- The preparation checklist is a prompt, not a specification. No quantities are worked out for it.
If a result looks wrong, we want to know: report a problem.
Rooms that are not a simple rectangle
- L-shaped room, walls: enter the longest length and the longest width. An L-shaped room has the same total length of wall as the rectangle it fits inside.
- L-shaped room, ceiling: split the L into two rectangles. Run the calculator for each one with only “Ceiling” ticked, add the two “paint needed” figures together, then round up to whole tins yourself.
- Chimney breasts and alcoves add wall that this version does not measure, so the estimate will be a little low. Each side return adds its depth × the room height: a chimney breast 0.35 m deep in a 2.4 m high room adds about 1.7 m² of wall. Work out the paint for that extra area and check it against the “left over” figure before you buy.
- Bay windows vary too much for a rule of thumb. Measure each section of wall and add it up by hand.
- Sloping ceilings and stairwells are not supported yet.
A worked example
A room 4.2 m long, 3.6 m wide and 2.4 m high, with one door 0.8 m wide and 2 m high and one window 1.2 m wide and 1.2 m high. The walls and the ceiling each get 2 coats of a paint that covers 10 m² per litre, with 10% extra, bought in 2.5 litre tins. These are generic example figures, not a product's: they are the numbers the calculator starts with.
Wall paint: 4 × 2.5 L tins
- Wall area before doors and windows 2 × (4.2 m + 3.6 m) × 2.4 m
- 37.44 m²
- Doors and windows taken off 1 × 0.8 m × 2 m + 1 × 1.2 m × 1.2 m
- − 3.04 m²
- Wall area to paint
- 34.4 m²
- Paint for 2 coats 34.4 m² × 2 ÷ 10 m² per litre
- 6.88 L
- Paint needed, with 10% allowance 6.88 L × 1.1
- 7.568 L
- Whole 2.5 L tins 7.568 L ÷ 2.5 L = 3.028, rounded up
- 4 tins (10 L)
- Expected left over 10 L − 7.568 L
- 2.432 L
Ceiling paint: 2 × 2.5 L tins
- Ceiling area to paint 4.2 m × 3.6 m
- 15.12 m²
- Paint for 2 coats 15.12 m² × 2 ÷ 10 m² per litre
- 3.024 L
- Paint needed, with 10% allowance 3.024 L × 1.1
- 3.327 L
- Whole 2.5 L tins 3.327 L ÷ 2.5 L = 1.331, rounded up
- 2 tins (5 L)
- Expected left over 5 L − 3.327 L
- 1.673 L
The walls and the ceiling are two separate paints, so their tins are not added together. Put your own room and the figures from your own tin into the calculator above: the coverage on the tin changes the answer.
Questions
Does the paint calculator take off doors and windows?
Yes, from the walls. Each door or window you add is taken off the wall area: its width times its height times how many. Nothing is taken off the ceiling. If you add none, the estimate comes out a little higher.
How many coats does it allow for?
It starts at 2 coats, as an example. Change it to the number the tin gives for your surface. The litres go up in step with the coats.
What coverage figure should I use?
The one printed on the tin or the product page of the paint you are buying, in square metres per litre for one coat. The calculator starts at 10 m² per litre, an example that is not any maker's figure. Coverage differs between products and surfaces, so the tin comes first.
Does the tin size change how much paint I need?
No, only how many tins. The calculator counts whole tins of the one size you choose (1, 2.5, 5 or 10 litres, or another size) and shows what it expects to be left over. In the example the walls need 7.568 L: that is 4 tins (10 L) at 2.5 litres or 2 tins (10 L) at 5 litres.
Why does it add 10% extra?
For waste and touch-ups. It is an example figure, not a rule. Change it, or set it to 0 to see the bare sum.