How to measure a window for replacement glass or a new sealed unit

Glass cannot be trimmed afterwards. A pane cut 5mm too big will not go in, and a sealed unit 5mm too small leaves gaps that let the weather in. Ten careful minutes with a tape measure saves a remake. This guide covers single panes, double glazed units and the photos that let a glazier check your numbers.

The short answer

Measure the width first, then the height, in millimetres, at three points each, and use the smallest. For a single pane, measure the rebate the glass sits in and take off a small clearance. For a double glazed unit, measure the existing unit with the beads off if you can, plus its thickness. Then send a photo with the tape in shot.

What you need

• A steel tape measure in millimetres. Cloth tapes stretch.
• A pencil and paper, or your phone's notes.
• For bead-glazed windows, a stiff putty knife to ease the beads out (optional).
• Gloves, if the pane is broken.

Know your glass first

What you have	How to tell	What to measure
Single pane in putty	Bevelled putty on the outside face	The rebate opening, then deduct
Single pane in beads	Wooden or plastic strips hold the glass	The rebate opening, then deduct
Double glazed unit	Two panes with a spacer bar between	The unit itself, plus thickness
Leaded or decorative	Lead lines or pattern in the glass	Photo first, ask before ordering

Not sure? The lighter test tells you how many panes you have in seconds.

Measuring a single pane

1. Remove any loose broken glass safely with gloves on, working from the top down. If you are not comfortable doing this, our glass cut to size service can do it.
2. Clean out the rebate so old putty or bead does not hide the real edge.
3. Measure the width of the opening at the top, middle and bottom, from the inside face of the rebate to the inside face of the rebate. Write down the smallest.
4. Measure the height at the left, middle and right. Write down the smallest.
5. Deduct the clearance, about 3mm from each measurement for a small pane, so the glass sits loose in the opening.
6. Check it is square by measuring the two diagonals. If they differ by more than a few millimetres, tell your glazier: the frame is out of square and the pane may need a template.

Write sizes width first, then height: width × height. Glaziers always read them that way round.

Measuring a double glazed unit

The most accurate way is to measure the unit itself.

1. Find the beads. On most uPVC windows they are on the inside; on some older units they are outside.
2. Ease out one long bead with a putty knife, starting in the middle. Leave the others in so the unit stays put.
3. Measure the unit edge to edge, not the visible glass. You will see the spacer bar and the black seal round it.
4. Measure the thickness straight across the edge of the unit. Common sizes are 24mm and 28mm.
5. Note the spacer colour (silver, black or white) and any georgian bars inside the unit.
6. Clip the bead back in.

If you do not want to remove beads, measure the visible glass, width then height, and send us the photos below. We work out the unit size from the frame system, and we always check a sealed unit on site before it is made.

The photos that make a quote accurate

• The whole window from inside, square on.
• A close-up of a corner showing the bead or putty and the edge of the glass.
• A tape measure held across the visible glass, width and then height, in shot.
• The stamp in the corner of the glass, if there is one: it tells us if it is toughened or laminated.
• Any marking printed on the spacer bar between the panes.

Worth knowing: doors, glass beside doors and low-level panes usually need safety glass by law, even if a plain pane was there before. Our guide to safety glass rules shows exactly which panes count.

Common mistakes

• Measuring in inches or centimetres. Glass is cut in millimetres. Convert before you order.
• Measuring the visible glass for a single pane. You will order it too small: the glass runs into the rebate.
• Not deducting clearance. A pane cut to the exact rebate size will not go in.
• Height before width. Swapped sizes are the most common remake we see.
• Ignoring the thickness. A sealed unit that is too thick will not take the beads back.

Let us check your numbers

Send your measurements and photos on WhatsApp and we will confirm the size and the glass type before anything is cut. You can collect stock glass from our Wood Green workshop, or we can measure and fit it for you. For sealed units, see misted double glazing repair. For anything broken right now, ring 07915 903485.