A replacement sealed unit is cut to the finished glass size. If that size is only the glass you can see, or the thickness is a guess, the new unit will not sit in the frame.
At The Glass Shop Online we make cut-to-size double glazed units every day. This is the measuring method we ask customers to use before we make anything.
Width and height
Measure from the back of the bead to the back of the bead, then take 10 mm off. Do the same for the height. If the back of the beads are 800 mm apart, the glass we make is 790 mm wide. That 10 mm is the clearance the unit needs to sit in the rebate.
The mistake we see most often is measuring only the glass that is visible. That size is too small. The new unit will not reach behind the beads, so it will not fit. Browse cut-to-size double glazed units.
Thickness
Glass calipers are the reliable way to get the thickness. On an opening window, place the calipers across both faces of the unit. That reading is the overall thickness we need.
If you do not have calipers, you can work it out from the frame. Measure the overall thickness of the frame, then measure from each glass face out to the outer face of the frame, and deduct both of those gaps. A 60 mm frame with 13 mm on the inside and 23 mm on the outside is a 24 mm unit, because 60 minus 36 is 24. A 28 mm unit is ordinarily 4 mm glass, a 20 mm spacer and 4 mm glass.
Taking the beads out and measuring the old unit is a last resort. It works, but it is more trouble than measuring in place.
What else to tell us
Size alone is not the whole order. Tell us whether you want clear glass, Low-E or a pattern, and what colour spacer bar you need. Say if the unit is a rectangle or a shape, and send a photo if you are not sure. We also need to know if the glass should be toughened or laminated, and whether it is going in a door or a window.
You need a tape measure and, ideally, glass calipers. If access is awkward or unsafe, we may be able to survey, depending on the area. It is better to ask than to order on a guess. A unit that is too thin will be loose. A unit that is too thick will not go into the rebate.
Width, height and thickness, plus Low-E, pattern, spacer colour and whether it is a door or a window.
Questions you might still have
Why deduct 10 mm?
Back of bead to back of bead is the opening. The unit needs clearance to sit in the rebate, so we take 10 mm off the width and the height.
What if I only measure the glass I can see?
The new unit will be too small to fit correctly behind the beads.
What if the thickness is wrong?
Too thin and the unit is loose. Too thick and it will not go into the rebate.