Measuring For A Window

If you want to make stained glass windows that are going to fit into a specific place, it’s important to get the measurements accurate right from the beginning. It’s best to take the measurements even before you start your design, so that you can scale the pattern up easily to fit.

Do you need to measure?

If you are learning how to make a stained glass sun catcher, or a hanging panel that doesn’t need to fit in a window frame, then you can skip the measuring and go straight to Designing or Finding a Pattern.

1. Measuring The Window Opening

This is quite hard to describe, but isn’t hard to do once you break it down logically.

If you think about the task in three stages, it might help. You need:

– the size of the opening – the Tight Size

– the adjustments needed to make room for putty – the Fitting Size

– the measurements needed so that the whole of your design is visible – the Sight Size

section drawing showing different measurements
Three measurements needed to measure a window

– the Tight Size = the exact size of the hole that you want to make your panel for.

Have a good look at the window or door panel. You can probably see that the sheet glass is held in with either putty or a wooden rebate. The drawing above might help. You need to measure not only the glass you can see, but to the edge of the frame underneath the putty or wooden rebate.

This will be your largest measurement.

– the Sight Size = the size of the glass area that you can actually see.

If you have beading or putty holding the existing glass in, measure to the edge of the beading. The diagram above explains this. You need to know this measurement so that when you make your panel, the whole of your design will be visible.

You wouldn’t want the tip of a leaf to be chopped off behind beading, you want it all to show.

Some people call this the Daylight Size.

This measurement is your smallest measurement.

2. The Fitting Size

Now you need to work out the third and most important measurement – the Fitting Size. This gives you the measurements for the outside edge of your stained glass panel.

If you imagine going to a glaziers and asking them to cut you a piece of glass for a window, they never cut the exact size of the opening, they always take a bit off around the edge so that there is room for the putty to fit in around it.

diagram of different measuring lines
Diagram showing three window measuring lines

– Work out the Fitting Size by subtracting 1/8th inch or 3mm from each edge of the Tight Size.

The drawing above might help you visualise what I’m talking about.

This set of measurements will be somewhere in between the Tight and Sight Size.

Once you’ve got your fitting size measurement, you can forget about the tight size altogether.

3. Things to Remember

– Make sure you measure all four sides. Some windows are not geometric!

– If you are making a pair of windows, measure both. Never assume they are the same size, especially in older buildings.

– Measure the angles to make sure the window is square.

4. Which Measurement Do I Use For Cutting The Art Glass?

Very good question. The answer is that you haven’t got that measurement yet…

You have the Fitting and Sight sizes, which are all you need to get started on your design.
But once you start making your pattern – or Cutline – for cutting the art glass pieces, you will need to take another set of measurements for the outside edge. If you look at the drawing below, you’ll see what I mean.

diagram with brown cutline detail
Drawing showing measurement for stained glass pieces

You can see from this diagram that you can’t use your Fitting size for cutting glass to, otherwise there will be no room for the lead or zinc came, and your panel will end up too big. So you must cut your glass smaller to allow for the border lead or zinc.

How much smaller depends on how wide your border lead is. Let’s use the example of 1/2inch (12mm) lead. This is the width that is usually used to edge window panels.

You want to cut your glass to the inside of the heart, so you need to measure 1/4inch (7mm) in from each edge of the Fitting size for your art glass cutting pattern (Cutline).

This Cutline will be in between your Fitting and Sight size.

Check Your Measurements

It seems very complicated when I explain it, but if you take it a step at a time, you’ll see that it’s quite logical and obvious. I always get all these measurements worked out – and checked three times! – at the beginning of a project. I write them all down in my sketchbook clearly, making sure I label them.

Once you’re happy with them, and are confident they’re accurate, you’re ready for Designing or Finding a Pattern.

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