Measuring a blown or misty sealed unit is simpler than most guides make it sound. All you need to do is measure the visible glass - the part you can see between the beads. Send us those two numbers and we add the 10mm tolerance ourselves, so the new unit slips into the frame with room for the gaskets and packers first time.
Why measuring accurately matters
A sealed unit is made to order. Once the two panes are bonded to the spacer bar and the perimeter is sealed, the size cannot be changed. Two minutes with a tape now is the difference between a 30-minute fit and an expensive do-over. Sealed units are bespoke and non-refundable if the size is wrong, which is why we'd rather you measure the easy bit (the visible glass) and let us handle the conversion.
What you need
- A metal tape measure (cloth tapes flex and lie)
- Pen and paper - write every measurement down immediately
- Two minutes per window
You do not need to remove the glass, and you do not need to remove the beading.
Step 1: Find the visible glass
Stand in front of the window and look at the pane. The clear area you can see - bordered by the angled PVCu bead on all four sides - is the visible glass (sometimes called the sight size). That's the only thing you're going to measure. Don't worry about the bead, the rebate, or any hidden glass tucked behind the bead - we account for all of that on our side.
Step 2: Measure the visible glass
Place the end of the tape on the edge of the visible glass on one side and run it across to the edge of the visible glass on the opposite side. Do this for both width (left edge to right edge) and height (top edge to bottom edge).
For example, if your visible glass width gives you 448mm and height gives you 865mm, write down 448 x 865mm. That's the number we need.
Tip: frames can sag slightly with age. Measure the width at both the top and the bottom, and the height on both sides, then send us the smaller of each pair. It only takes another 20 seconds and avoids a tight squeeze on an out-of-square frame.
Step 3: Measure the unit thickness
The thickness of the existing unit must match exactly or the beads won't clip back on properly. Carefully measure from the outer face of the outer pane to the outer face of the inner pane. Common UK sealed unit thicknesses are:
- 20mm - older builds, often 4mm-12-4mm (4mm glass, 12mm spacer, 4mm glass)
- 24mm - the modern standard, usually 4mm-16-4mm with argon and a Low-E coating
- 28mm - higher performance, sometimes triple glazed
If you're between sizes (e.g. 22mm), it's almost always 20mm with paint or sealant adding a little. When in doubt, send us a photo and we'll confirm it.
Step 4: Send us the numbers
That's it on your side. Send us the visible glass width, the visible glass height, the thickness and (if you can) a quick photo of the window. We add the 10mm all-round tolerance ourselves - that's the gap the sealed unit needs for the rubber gaskets, plastic packers and the small amount of thermal movement PVCu makes through the year. Different frame materials need different tolerances (timber and aluminium are slightly less than PVCu), so just tell us the frame type and we'll set the right number.
The fastest way to get a price back is our instant quote calculator - punch the visible glass dimensions in and it gives you a fixed total in seconds.
Common mistakes to avoid
- Measuring the frame opening or the brick reveal. Stick to the visible pane - the bit of clear glass you can see between the beads. Anything bigger and the unit won't fit; anything smaller and the beads won't hold it.
- Deducting anything yourself. Don't. Send us the raw visible-glass figure - we add the tolerance. If you also deduct, the unit comes back 20mm undersized and unusable.
- Measuring only one side. Frames can sag, especially older ones. Take the width at the top and bottom and the height on both sides; send the smaller of each pair.
- Guessing the thickness. A 4mm difference in unit thickness means the beads won't go back on. Always measure.
- Not noting decorative features. If the unit has obscure / patterned glass, leaded lights or Georgian bars, mention it - those units are templated, not measured.
Prefer we just measure it for you?
For most of our Essex customers, that's the easier route. We come out, measure every blown unit on the property, quote on the spot and book the fit. No risk of a wrong-size unit, no waste, and our glass is covered by a 25-year guarantee that DIY-supplied units are not. Use our free quote calculator for an instant ballpark, or book a survey via the contact page.