How do I measure a replacement double glazed unit?

A sealed unit is made to order and cannot be trimmed. Everything depends on four measurements and a short specification, and getting the thickness wrong is the mistake that costs a second unit.

What you'll need

• A steel tape measure
• A glass thickness gauge, or a ruler and a torch
• A bead lifter
• A camera

Step by step

1. Take a bead out if you possibly can.Measuring the actual unit is far more reliable than adding an assumed bead overlap. One short bead released at the centre gives you access to the edge of the glass.
2. Measure width and height at three points each.Top, middle and bottom for the width; both sides and the centre for the height. Take the smallest of each. Frames are rarely square.
3. Deduct a clearance.Most fabricators want the unit 3–5mm smaller than the rebate in both directions so it sits on packers with room to move. Tell them the rebate size and let them apply their own tolerance.
4. Measure the thickness accurately.A glass gauge is the reliable way. 24mm and 28mm overall are the common domestic sizes, made up as 4mm glass, a spacer, and 4mm glass. Getting this wrong means the beads will not fit.
5. Count the panes.Hold a lit match or a pen light against the glass at an angle and count the reflections. Two panes give four reflections; triple glazing gives six.
6. Check for a low-emissivity coating.The same reflection test shows one image with a slightly different colour if there is a low-E coating, and the coating faces into the cavity. It affects performance and should be matched.
7. Note the spacer bar.Colour and type — aluminium, or a warm-edge composite. Warm-edge spacers reduce condensation at the glass edge and are worth specifying even if the old unit had aluminium.
8. Identify any toughened or laminated glass.Look for an etched safety mark in a corner. If the old unit was toughened or laminated, the replacement must be too — it is usually there because the location legally requires it.
9. Note any pattern, tint or Georgian bar.Obscure glass patterns have names, and there are dozens. Photograph it and take a sample if you can, because matching by description alone rarely works.
10. Order and check on arrival.Measure the unit before the fitter leaves or before you pay. A unit a few millimetres oversize cannot be adjusted.

Tips

• Write the measurements as width first, then height, and say so explicitly. Transposed dimensions on a non-square unit are a very common and very expensive error.
• Photograph the etched safety mark and any spacer branding. It tells the fabricator more than a description will.
• If several units in the house have misted, measure them all at once. Fabricators price per unit and the labour to fit is much lower done together.

Common mistakes

• Measuring the visible glass and ordering that size — The bead covers 8–10mm of glass on each edge, so a unit ordered to the sight size is 16–20mm too small in each direction. It rattles in the rebate and cannot be sealed.
• Guessing the overall thickness — A 28mm unit will not go into a rebate for a 24mm unit, and a 24mm unit in a 28mm rebate leaves the beads unsupported. It is a five-minute measurement that prevents a total loss.

Questions people ask