How to measure uPVC windows

October 17, 2018

It’s common knowledge that uPVC windows are some of the most affordable and efficient windows on the market, but if you’re looking to replace your current windows, you might not know how to measure the existing ones in preparation for the new ones. If this is the case, don’t fret as it’s relatively simple to work out.

Measuring the standard uPVC window

Rosewood uPVC casement window

Rosewood uPVC casement window

It should immediately be noted that you should measure your window from the outside of your home as this will give you a more accurate depiction of the window’s size. The measurements need to include the window’s frame, the windowsill, and if you have any frame extensions, these should be included too. You need to measure render to render (the area holding your window in place) as opposed to just measuring the window itself.

You’ll have to measure the window in three different places, for both height and width. To measure the height, you’ll want to measure the left side, the right side, and the centre of the window. To measure the width, you should measure the top, bottom, and centre of the window. After doing this, use the smallest height for both height and width respectively, then deduct 10mm from both, this is done to allow for manufacturing tolerances and also to account for any expansion that might occur due to hot weather.

Measuring a uPVC square bay window

If you’re measuring a square bay window, it has to be done slightly differently. This is because they’re less ‘flat’ and have three sides with corner posts. To measure the width, you’ll need to measure internally edge to edge from the frame. As a square bay window has three sides, you’ll need to measure each side window internally and ensure they’re the same size.
To measure the height, you just need to measure render to render, including the windowsill. Your corner posts should be 90 degrees, making them 70mm square each so you’ll need to add a total of 140mm onto the external size.

Measuring a uPVC bow window

A bow window is very unique in its shape and design, meaning the way it’s measured is also unique. They usually consist of more than two sides and have shallower angles. The width needs to be measured internally edge to edge from the existing frames. Bow windows are made up of smaller individual windows, each one will need to be measured separately and internally, from one edge of the frame to the other. To measure the projection of the bow window, you need to measure internally from the back span to the inside point of the window frame.

Jack of Glass

At Jack of Glass, we offer a wide range of windows, doors, and other glass products. We also have a repair service, capable of tackling a variety of problems. For more information, get in touch with us or browse our website.


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Operating out of High Wycombe for 30 years, we know the local area like the back of our hand. We supply and install high quality glass and glazing to homes in Princes Risborough, Chesham, Amersham, Beaconsfield, Marlow and beyond.

If you have a particular way of improving your home in mind and reside anywhere in or around a 20 mile radius, feel free to get in touch with our reliable team. We’ll be in touch swiftly.

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M. Shoaib

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Josh Webb

Jack of glass have recently replaced all my windows efficiently and to a high standard. The price was also very reasonable. The customer service was outstanding along with the services they provide. Highly recommended.

Mr Bennett

I had to replace a small double glazed panel in a window. I though it would be difficult to get anybody interested as it was such a small job. However, Jack of Glass did the work. They visited the house to take the measurements, ordered the replacement glass and fitted it all very efficiently and at a reasonable cost. I will use them again.

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