Guide · Mirrors
How to measure for a mirror (or a shelf) in millimetres
A step-by-step way to measure a wall, a recess or an old frame for a cut-to-size mirror or glass shelf, and how much clearance to leave.
Glass is cut to the millimetre and cannot be trimmed once it arrives, so the measurement you type in is the piece you get. Ten minutes with a tape measure saves a remake.
What you need
- A steel tape measure that reads in millimetres. Fabric tapes stretch.
- A pencil and paper. Write every number down as you go.
- A spirit level if the mirror is going on a wall.
1. Decide which way round it goes
Our calculator asks for width and length. For a mirror, treat width as side to side and length as top to bottom. The price is the same either way, but writing it down this way avoids mix-ups when you hang it.
2. Measure three times in each direction
Walls, recesses and cabinets are rarely square. Measure the width at the top, middle and bottom, and the height at the left, centre and right.
Use the smallest number from each set. A piece cut to the largest number will not fit the narrowest point.
3. Take off clearance where the glass fits inside something
If the mirror or shelf sits inside a recess, a niche, a cabinet or an old frame, it needs a small gap so it goes in without being forced.
- Recess or niche: take about 3 to 5mm off each dimension.
- Cabinet shelf: take 2 to 4mm off the width and a few millimetres off the depth so the door still closes.
- Replacing glass in a frame: measure the rebate (the step the glass sits in), then take 2 to 3mm off each dimension.
If the mirror is simply going on a flat wall, there is nothing to take off. Choose the size you want to see.
4. Check what is in the way
Power points, light switches, taps, skirting boards and door frames all eat into a wall. Mark them on your sketch with their distance from the edges. Glass through the calculator is a plain rectangle or circle; if you need a notch or cut-out, email us a sketch instead.
5. Round mirrors
Enter the diameter: the distance straight across the middle. If you are replacing a round mirror, measure it in two directions at right angles. If the numbers differ it was not quite round; use the smaller one.
6. Heavy is heavier than you think
Mirror weighs about 10kg per square metre at 4mm and 15kg at 6mm. A 1200 × 1800mm mirror in 6mm is over 30kg. Use fixings rated for the weight and the wall you are fixing to.
Before you order
Read your numbers back against the sketch. Toughened pieces in particular cannot be altered once made.