How to measure a room

Almost every problem with a room plan traces back to the measuring, not the drawing. Get the numbers right and everything downstream — flooring quantities, whether the sofa fits, what the joiner quotes — follows. Get them wrong and you find out when a box of flooring runs out three planks from the wall.

What you need

A tape measure long enough to span the room in one go — 5 m is enough for most rooms, 8 m is more comfortable. A laser measure is faster and more accurate for long spans, and cheap ones are fine for this. Paper and a pencil beat a phone: you will be writing numbers while holding a tape.

Sketch first, measure second

Draw the room roughly before you measure anything — not to scale, just the shape, with every wall, door, window, radiator and alcove marked. This does two things: it forces you to notice features before you are crouched in a corner with a tape, and it gives you somewhere to write each number as you take it.

Number the walls as you go round, clockwise from the door. When you later come to draw the plan properly, working round in a consistent direction is what stops you flipping the room by mistake.

Measuring the walls

1. Sketch the room roughly before you measure anything, marking every wall, door and window.
2. Measure each wall at floor level, corner to corner, and write the number straight onto the sketch.
3. Measure the room's two diagonals to check whether it is actually square.
4. Measure each door and window: width, height, and the distance from the nearest corner.
5. Measure the ceiling height in two places, and note anything that eats into the room.

Measure at floor level, not at waist height. Walls bow, and skirting boards and pipe boxing change the usable width — floor level is where the flooring goes and where furniture actually sits.

Rooms are not square, and the diagonals prove it

In a genuinely rectangular room the two diagonals are equal. Measure both. If they differ by more than a centimetre or so, the room is out of square, and that matters: tile and plank layouts started from a wall that is not parallel will visibly drift by the time they reach the far side.

Where the room is out of square, measure each wall at both ends as well as in the middle. Use the largest figure when ordering materials and the smallest when checking whether something fits.

The things people forget

• Door swings. A standard 826 mm door needs 826 mm of clear floor to open into. That is a piece of the room you cannot furnish.
• Radiators. They project 60–100 mm and need air, so nothing goes tight against them.
• Chimney breasts and alcoves. Measure the depth of the breast and the width of each alcove separately — the alcoves are rarely the same width.
• Skirting. Furniture sits against the skirting, not the plaster, which costs you 15–25 mm on every wall.
• Window sills and reveals. Relevant the moment anything tall goes near a window.

Write it down in millimetres

Use whole millimetres, not centimetres or decimal metres. Every trade drawing in the UK is dimensioned in millimetres for one reason: whole numbers cannot lose a decimal point. 3658 is harder to mistype than 3.658. If you are working in feet, convert once at the end rather than repeatedly — the feet to metres converter will do it.

Check the numbers close

Before you put the tape away, add up the walls and check the shape works. In a rectangle the opposite walls should match. In an L-shape, the two short sections of the L should add up to the long wall opposite them. If they do not, one measurement is wrong, and it is much cheaper to find that now than after ordering.

This is the check the planner does for you automatically: type each wall in as you go and the room either closes back on itself or it visibly does not, which tells you immediately that something needs re-measuring.

Then work out what you need

With the numbers in hand: square footage for the room size, flooring for boxes to order, and paint for litres.