How to measure a room properly
Every calculator on this site is only as good as the numbers you type into it. Most material shortfalls are not calculation errors — they happen before any arithmetic starts.
Your room is not rectangular
Start from this and everything else follows. Almost no room is square, level or parallel. Walls bow, corners are rarely 90°, floors fall away, and a wall that measures 3.60 m at skirting level often measures 3.63 m at the ceiling. None of that is a defect — it is normal construction tolerance. It only becomes a problem when you measure once, in one place, and order to that figure.
Measure each wall in three places
Near the floor, at about waist height, and near the ceiling. Write down all three. Then:
- For anything you are covering — tile, flooring, paper — use the largest figure. You cannot stretch material to fit a wall that turns out wider than you thought.
- For anything you are fitting between two walls — a worktop, a shelf, a bath panel — use the smallest. Anything cut to the widest point will not go in.
The gap between those two figures is the room's error, and it tells you how carefully you need to work. Under 5 mm across a wall, ignore it. Over 20 mm, you are going to be scribing things to fit and you should plan for it now.
Check whether the room is square
Two ways, both quick:
- Compare the diagonals. Measure corner to opposite corner, both ways. In a true rectangle they are identical. The difference is how far out of square you are.
- The 3-4-5 check. From a corner, mark 3 units along one wall and 4 along the other. The distance between those marks is exactly 5 units if the corner is a right angle. Works in any unit — 30 cm, 40 cm, 50 cm does the same job as 3 m, 4 m, 5 m.
Out of square matters most for anything with a visible grid. A tiled floor set out square to one wall will show a tapering cut against the opposite wall, and a 20 mm error across a room becomes a wedge that goes from nothing to 20 mm along a single row of tiles — obvious once you see it, impossible to unsee.
Where to hide the error. You cannot remove it, only decide where it goes. Put the tapering cut where nobody looks: under a kitchen unit, behind a door as it swings open, along the wall furthest from the window. Never put it along the most visible wall or across a threshold.
Measure what you are actually covering
Floors
Measure the widest point in each direction, including anything the floor runs into: a doorway threshold, a bay, an alcove, the recess under a stair. Those are all floor area and they are all easy to forget because they are not part of the main rectangle.
For an L-shaped or irregular room, split it into rectangles on paper, measure each one, and add them. Do not try to average an awkward shape into a single rectangle — you will be wrong, and the direction of the error is unpredictable.
Walls
For paint, you want total wall area, so measure the perimeter and multiply by the height. For wallpaper you want the perimeter as a single running figure, because the roll is divided into drops across it — area is the wrong unit entirely. For tiling you want each wall separately, because you tile each one to its own height.
Measure the height from the finished floor, not from the subfloor. If you are laying 20 mm of flooring later, the wall you paper is 20 mm shorter than the one you measured.
Openings
Measure them, always, even when you have decided not to deduct them. The decision to deduct or not depends on the material — the rules differ for paint, tile and wallpaper, and are explained on each calculator — but you cannot make that decision later without the numbers.
The mistakes that actually cost money
| Mistake | What it costs |
|---|---|
| Measuring plasterboard before it is skimmed | 2–3 mm per wall, which matters for anything fitted between walls |
| Measuring to the skirting instead of the wall | 15–25 mm each side — enough to change a tile layout completely |
| Forgetting the doorway threshold | A missing strip of floor exactly where everyone walks |
| Using the architect's drawing rather than the built room | Drawings are intentions. Buildings are what got built. |
| Measuring a bay as a rectangle | Under-orders by the area of the two triangles, typically 10–20% |
| Rounding each wall down "to be neat" | Four small roundings compound into a shortfall |
| Not writing it down immediately | You will misremember 3.64 as 3.46. Everyone does. |
Tape or laser
A laser is faster, works alone, and is more accurate over long distances where a tape sags. It is the better tool for room dimensions and for wall heights.
A tape is better for anything under about a metre, for measuring around obstacles, and for anything where you need to feel the corner. Lasers also struggle with dark, glossy or mirrored surfaces, and they measure to whatever the beam hits — including the picture frame you forgot was on the far wall.
Use both. Laser for the room, tape for the details. If you have only a tape, hook it on the far wall rather than holding it loose, and keep it flat on the floor rather than spanning across the middle of the room.
Record it so it is still useful tomorrow
Sketch the room, roughly to shape but not to scale, before you measure anything. Then write each figure onto the sketch as you take it. This takes two extra minutes and eliminates the single most common problem in the whole process: a list of numbers with no reliable memory of which wall each one belongs to.
On the sketch, mark:
- Every wall length, with all three heights where they differ
- Ceiling height, measured in at least two corners
- Every opening, with its width, height and the distance from the nearest corner
- Which way any door swings
- Both diagonals
- Anything fixed that you are working around — a radiator, a boiler, a chimney breast, a soil pipe
Photograph the sketch. You will want it in the merchant's car park.
How much slack to leave
None in your measurements — measure accurately and record what you actually found. The allowance belongs in the waste factor, not in the dimensions, and the waste factor table already handles it by material and pattern. Padding your measurements as well as applying a waste factor means you are compounding two allowances and over-ordering, and it makes the figures useless for the next job.
Use it
Once you have a sketch you trust, the numbers go straight into whichever calculator applies:
- Tile — per wall or floor, plus the deductions you decided on
- Flooring — length and width separately, so the last-row check works
- Paint — perimeter, height and opening count
- Wallpaper — total wall run as a single figure
Last reviewed 2026-09-18.