Every one of these jobs needs a different measurement, and getting the wrong one is how people end up two boxes of tiles short on a Sunday. Paint needs wall area minus the openings. Flooring needs floor area plus waste. Wallpaper needs wall width rather than area. They are not interchangeable.
Measure it once, properly
- Length and width at floor level, in the same units throughout. Mixing feet and inches with metres is the single most common source of error.
- Ceiling height, measured in two or three places. Older houses are rarely square, and the shortest measurement is the one that matters for wallpaper drops.
- Every door and window — width and height of each.
- Anything that breaks the wall: a chimney breast, an alcove, built-in wardrobes.
For anything but a plain rectangle, split the room into rectangles, calculate each, and add them. An L-shaped room is two rectangles. This is much more reliable than trying to be clever with the overall footprint.
Floor area — flooring, carpet, underfloor heating
Length × width, in the same units. That is your floor area, and the square footage calculator will do it in whichever units you measured in.
Then add waste, which is not optional:
- Straight-laid planks or tiles — add 10%.
- Diagonal or herringbone — add 15–20%, because every edge piece is a cut.
- Patterned carpet or vinyl — add enough to match the repeat, which can be substantial.
- An awkward room — more alcoves means more cuts means more waste.
Buy it all at once. Batch numbers matter: the same product from a different batch can be visibly a different shade, and it is the reason “I’ll get more if I run out” goes wrong. The flooring calculator takes the waste percentage and works in packs, since that is how it is actually sold.
Wall area — paint
Add up the wall widths — that is the perimeter — and multiply by the ceiling height. Then subtract the doors and windows, which people routinely forget. A standard door is around 1.8 m², a typical window 1.5 m²; in a small room with two of each that is a fifth of the wall gone.
Two things that surprise people about paint:
- Coverage is per coat. A tin saying 12 m² per litre means one coat. You almost always need two, and three over a colour change or bare plaster.
- Bare plaster drinks the first coat. A mist coat — watered-down emulsion — is a separate quantity on top.
The paint calculator handles coats and openings together, which is where hand arithmetic usually goes astray.
Wall width — wallpaper
Wallpaper is the one that is not an area calculation, and treating it as one is why people under-order. It comes in rolls of fixed width, so what matters is how many full drops you can cut from a roll.
A standard roll is 10.05 m long and 53 cm wide. With a 2.4 m ceiling you get four drops per roll — 9.6 m used, with 45 cm left over that is no use to anyone. Divide your wall width by 0.53 to get the number of drops, then divide by drops-per-roll.
Patterned paper needs more. Each drop has to be cut so the pattern lines up, and the offcut is wasted. A 64 cm repeat can cost you a drop per roll — closer to 20% more paper, not 5%. The wallpaper calculator accounts for the repeat rather than assuming it away.
Tiles
Area again, but the waste allowance is higher because every edge is a cut: 10% for a simple rectangular room, 15% if it is awkward or the tiles are laid diagonally. Add a few spare beyond that — tiles get chipped during fitting and, again, batches vary.
For a wall, measure each wall separately rather than totalling. You rarely tile all of them to the same height, and a single number hides that. The tile calculator works in tiles and boxes, which is what you actually buy.
Volume — plaster, screed, concrete
Area × depth, and the depth is where the mistakes live because it is in millimetres while everything else is in metres. A 50 mm screed over 20 m² is 20 × 0.05 = 1 m³, not 20 × 50.
Concrete is sold by the cubic metre and always over-order slightly — ground is never perfectly level, and a short pour cannot be fixed later. The concrete calculator works in cubic metres and bags.
The short version
| Job | Measure | Add |
|---|---|---|
| Flooring | Floor area | 10%, or 15–20% diagonal |
| Paint | Wall area − openings | × number of coats |
| Wallpaper | Wall width | Extra for the pattern repeat |
| Tiles | Area per wall | 10–15% |
| Concrete | Area × depth | 5–10% |
If you are measuring in feet and buying in metres, or the other way round, metric and imperial covers the conversions and the ones that catch people out.