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.