How to Measure The Window When Learning How To Replace Window Glass

it is important to know the process of how to measure glass and today i want to talk to you about how to measure glass in a double glazed window.

Double glazed windows can be either made of wood or of upvc, and i want to take a look at how to measure glass in a upvc window. Upvc windows can either be what is known in the trade as internally or externally glazed, meaning that the glass either fits into the window frame from either the inside or the outside of the property.

Either way the upvc window will have what is called a glazing bead, this is a plastic bead which runs around the perimeter of the glass and will either be seen on the inside or the outside of the window frame.

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The glazing bead can be noted by the fact it is clearly identified as being a seperate peice of plastic apart from the main frame of the window.

Once located here is what you need to do. You will see that on the side of the bead which actually touches the glass there is a gasket which fits tight upto the glass pane, this gasket is to prevent any water ingress behind the glazing bead, and on the other outer line of the glazing bead you will see a faint line or joint between the bead and the main window frame, this can be seen at both sides of the glass and at the top and bottom of the glass as the bead is the same all the way around the glass.

What you need to do is to take a tape measure and place the end of the tape onto that joint on the left hand side of the window, extend the tape until you get to that same joint at the right hand side of the glass pane and take that measurement, these two points are the extreme points of the opening when the beads are not in place so if the glass was to be made at this size it would not fit, so what you need to do is now deduct 10mm from this measurement, now measure from that same joint at the top of the glass and down to the bottom of the glass and again deduct 10mm from this measurement also. Because you haver deducted 10mm from the width and height measurements you can ensure that the new glass will have a 5mm clearance all the way around for ease of installation.

It is important to remember that knowing how to measure glass on the width and the height of your upvc double glazed window is only half of the process as you also need to know the overall thickness of the double glazed glass.

The supplier of the Double Glazing panes also need to know the overall thickness of the Double Glazed unit . There are two ways of establishing the thickness of the overall pane (No 1 is very expensive and can be only justified if you are going to replace a substantial number of them).

1) There is a kit that you can use that has several instruments in; two of them can indicate what type of glass you are looking at and the third one measures the thickness of the glass pane. One such supplier is Merlin Lazer and is called the Merlin Lazer Glass Analysis kit (approx £300 plus VAT). Measurements can be taken simply by placing the instruments up to the glass pane and taking the measurements.
Merlin Lazer contact details are as follows:  tel: (01892) 654141 or website is
2) The installer can take the unit into a glass supplier (these can be found in Yellow Pages or Thomson Directory in the UK or under ‘glass suppliers’ on Google. They could then measure it for you.

The glass itself is nearly always 4mm thick hence the two panes are 8mm. The space in between can be 6mm, 8mm, 10mm, 12mm, 14mm, 16mm, 18mm or 20mm.
Hence if you are looking at a Double Glazed unit that has a spacer of 12 mm, the overall thickness is 12mm (space) plus 8mm (glass) totalling 20mm thickness and this is expressed as 4 12 4.
If you are looking at a Double Glazed unit that has a spacer of 16 mm, the overall thickness is 16mm (space) plus 8mm (glass) totalling 24mm thickness and this is expressed as 4 16 4.
Diagram 2 – An Example of A Window System

It is very important to know how to measure glass properly especially when it is double glazing as there is not much tollerence to play with so you have to get it right.

It is however, always recommended that you use a qualified Window fitter to replace your Double Glazed (DG) panes. Even though a straightforward task, if you do your own, you undertake the task at your own risk.

If you are fed up with staring at a misted up or broken Window or Double Glazed unit, please click REPLACE DOUBLE GLAZING to grab a FREE 19 Page Report and you will get EVERYTHING you need to know to replace you Double Glazed units TODAY.