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How To Measure Windows For Replacement

Posted: October 20th, 2022

If you have decided to remove your existing windows and invest in replacement windows, then you are probably wondering how to measure windows accurately to ensure that your new windows are the correct size.

When measuring windows it is imperative to ensure you have accurate measurements to ensure a smooth window installation. If you purchase windows with incorrect measurements, this could delay your whole project and you will have to go through the process again.

What tools do you need?

Before you begin collecting your height and width measurements, it is important that you have the right tools. To get the correct measurements for your replacement windows, you will need a high-quality tape measure, a pencil, a notebook and a ladder if your upper windows are a different size to your lower windows.

When writing down your measurements, you will need to write them as height x width.

Top tips before you get started

There are a few tips that you need to be aware of before you start measuring windows. Firstly, measure replacement windows in mm as this is the standard metric that is used across the industry.

When measuring your windows, make sure that your tape measure is straight and taught throughout your measuring process. This will ensure that you have accurate measurements.

Furthermore, remember the golden rule - measure twice cut once. Measure all of your windows at least twice to make sure your replacement windows are the correct size.

Moreover, use the 10mm deduction rule. When installing uPVC windows, you need to leave a tolerance gap to allow a small fitting allowance to ensure the new window frame sits plumb once it has been installed. This also accommodates for natural shifting over time. Therefore, when you measure replacement windows, it's best practice to deduct 10mm from both the final height and width measurements to leave a tolerance gap.

However, you need to be careful with this rule as having too little or too much tolerance can have a negative impact. Having too little tolerance could lead to the window not sitting in the gap correctly after the sealant has been applied. It also doesn't give the window much room to naturally shift over time. On the other hand, having too much tolerance could leave gaps which will decrease the energy efficiency of the window. When using this rule, make sure you have measured your windows at least twice beforehand so you have an accurate measurement.

How to measure for replacement windows

Now that you have the tools you will need and you have read through the tips before getting started, it is time to get into how to measure windows. You can use this method for most standard window frames such as uPVC windows, casement windows and flush windows.

It's important to note that you need to take down the external window measurements to present to the manufacturer. You need to ignore the old window frame as this may have shifted and become misshapen over time. Instead, measure brick to brick from the outside of each window. When you measure replacement windows, include the window sill and any frame extenders within the measurements.

How to measure the width of the window

For the width measurements, you need to take three external width measurements. Horizontally measure the top, middle and bottom from brick to brick. Once you have your measurements, you will need to present the smallest measurement to the manufacturer for your replacement window.

How to measure the height of the window

For the height measurements, you also need to take three external height measurements. Vertically measure the length of the right-hand side of the window, the centre and the left-hand side of the window. Similarly to the width measurements, you will need to present the smallest measurement from your final height measurements to the manufacturer.

Time for you to invest in your replacement windows

Now that you know how to measure windows, you can collect the correct measurements for your windows. Once you have the measurements, you can send your height and width measurements off to uPVC window suppliers and receive a quote for your windows. You can use our easy window and door builder to input your measurements and get a quote.

After you have found the DIY uPVC windows that you want, all you need to do is find a tradesman to install them or do this yourself.

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Why Choose Essex Trade Windows


Direct from the Factory

Essex Trade Windows have been manufacturing UPVC windows & Doors for over 15 years. We manufacture to the highest quality as well as offering low prices & short lead times.


Genuine Trade Prices

We offer genuine trade prices & discounts to all of our installers, tradesmen and DIY customers.


10 Year Guarantee

Our supply only UPVC windows & doors come with a 10 year guarantee including frames, glass and hardware as standard.


Made to Measure

All of our products are made to measure to your exact requirements. Due to the vast range of options and finishes there’s sure to be a solution for every project.


Environmentally Friendly

Our Aluplast Ideal 70 windows & doors feature Ecotech70 as standard making it one of the most sustainable window and door systems in the UK.


Quality Products

Quality components from leading brands and fabrication by our highly experienced workforce ensure the highest level of quality.

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