How to Measure a Room Properly

Last updated: 23 August 2026

Measure each wall face to face at waist height, working clockwise. Take the full run of every wall first, then the parts within it — the parts must sum to the whole, and when they don't you have caught an error while still standing in the room. Then measure both diagonals. In a true rectangle they are equal; they almost never are, and how far apart they sit is the single most useful number you will take away.

A room takes about six minutes to measure properly. Doing it twice because the plan would not close takes twenty.

What to measure with

A laser distance meter, if you are measuring more than three rooms. They cost around €25–40, read to a couple of millimetres, and remove the two things that make tape measurements unreliable: sag over long runs, and needing a second person.

A tape measure works for a single room. Use a 5 m or longer one, keep it taut, and get someone to hold the far end for anything over 4 m — a sagging tape reads long, and it reads long by more than you would guess.

What you also need: paper, a pencil, and a rough sketch of the room drawn before you measure anything. The sketch is not the plan. It exists so every number you take has somewhere to land immediately, instead of being held in your head while you climb over a sofa.

Measure in this order

Order matters because each step catches errors in the one before it.

1. The full run of every wall

Go clockwise, starting at the door. Write each number on your sketch as you take it, against the wall it belongs to. Clockwise is not superstition — it means you never have to work out which wall you have already done.

Measure face to face at about waist height. Skirting boards, coving, uneven plaster and radiator pipes all lie to you near the floor and the ceiling. Waist height is the honest part of the wall, and it is also where furniture actually meets it.

2. The parts within each wall

For any wall carrying a door, a window, a chimney breast or an alcove, measure each segment: corner to opening, the opening itself, opening to next corner.

The parts must sum to the full run. If they do not, one of the numbers is wrong and you are still in the room to fix it. This is the cheapest error check in the whole process and most people skip it.

3. Both diagonals

Corner to opposite corner, twice. This is the step this article exists for, and it has its own section below.

4. The ceiling height

In each room, and in more than one spot if you have any reason to doubt it — extensions, loft conversions and older houses routinely vary by 50 mm or more across a single room.

5. The wall thickness

Measure the depth of a doorway reveal, from face to face. That is the wall's thickness and it takes five seconds. Guessing it is the most common reason a whole-floor plan fails to close.

The diagonal check

In a true rectangle, both diagonals are exactly equal. Measure corner to opposite corner one way, then the other. Whatever the difference is, that is how far from square your room sits.

Almost no room is square. A 10–20 mm difference is normal in any building. 50 mm or more means the room is visibly out of true and you need to decide what to do about it.

Why it matters more than it sounds. If you draw an out-of-square room as a rectangle, the error does not politely stay in that room. It propagates along every wall that room shares with the next one, and by the far side of the house your plan is 100 mm wider than the building. That is the point at which people conclude they measured everything wrong and start again — when in fact they measured fine and drew wrong.

What to do with the number:

Difference	What to do
Under 20 mm	Ignore it. Draw the rectangle.
20–50 mm	Draw the rectangle, but note it — it will matter for fitted joinery and for nothing else.
Over 50 mm	Draw the true four-sided shape. Most floor plan software has a free-form or polygon tool for exactly this.

In short: four extra measurements per room tell you whether your drawing can be trusted at the far end of the house.

Doors and windows need three numbers

Most people take one — the width — and then cannot place the opening when they come to draw.

The width of the structural opening, not the door leaf.
The offset: from the nearest corner to the edge of the opening. Without it you know a door exists but not where, and a door 200 mm out is the difference between a sofa fitting and not.
The sill height, for windows: floor to the bottom of the opening. This decides whether a desk, a radiator or a worktop can go under it.

Also note which way each door swings, and into which room. A door opening into a small bedroom sterilises roughly 0.8 m² of it, and that is frequently the fact that decides a layout.

For reference, common interior door widths: 730 / 830 / 930 mm in France and much of continental Europe, 762 mm (2 ft 6 in) in the UK, and 32 inches (813 mm) in the US, which is also the usual accessibility minimum. If your measurement is within a few millimetres of one of these, it is probably that one and your tape was slightly off.

Five mistakes that waste an afternoon

Measuring at floor level. Skirting typically adds 15–20 mm per side and turns a 3,000 mm room into 2,965 mm.
Mixing units. Work in millimetres or centimetres throughout. Mixed units and imperial fractions are where arithmetic errors hide.
Rounding as you go. Write what you read. Round once, at the end, if at all.
Not measuring the wall thickness. Assumed thickness is the most common reason room widths plus walls do not equal the outside of the building.
Trusting the estate agent's floor plan. It was drawn to sell the property, often from a quick capture, and its dimensions are frequently approximate. Measure it yourself.

Check it closes before you draw

Add up the room widths plus the wall thicknesses along one straight run of the building, and compare against the exterior measurement of that same run.

They should agree within about 50 mm. If they do not, it is almost always one of three things: a wall thickness you assumed rather than measured, a room drawn as a rectangle that is not one, or two measurements taken in different units.

Do this before you furnish anything. Every error left in place gets multiplied by every decision made on top of it.

Frequently asked questions

How long does it take to measure a room? About six minutes for a plain rectangular room with one door and one window, using a laser meter. A room with alcoves, a chimney breast or several openings takes ten to fifteen. Measuring an average three-bedroom house is roughly 90 minutes.

Should I measure in metric or imperial? Whichever your building was built in, and never both. Metric is easier because everything is decimal and errors are visible. If you are in the US, work in inches throughout rather than feet-and-inches, for the same reason.

Do I measure from the wall or the skirting board? From the wall, at about waist height. The skirting board is not the wall, and including it makes every room 30–40 mm smaller than it is.

How accurate does a room measurement need to be? For layout and furniture planning, ±50 mm across a room is fine. For fitted joinery — kitchens, wardrobes, bookcases — you need ±5 mm and you should measure that run three times. For anything structural you need a professional survey. The key difference is that layout tolerates error and joinery does not.

Can I just use a phone app to scan the room? LiDAR scanning on recent iPhones and iPads gives a usable rough plan in minutes and is genuinely good for a first pass. It is not reliable to the millimetre, it struggles with glass and mirrors, and it does not remove the need for the diagonal check. Treat a scan as a fast first step, not a finished measurement.

What if the room is not a rectangle at all? Measure it as a series of straight runs, taking every corner in turn, and note which corners are internal and which external. Then draw it with a free-form or polygon tool. Do not try to force an L-shaped room into two rectangles unless they genuinely meet at a square corner.