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How to Measure for Windows: A Short Guide

Buying new or replacement windows can be a difficult process, especially if you are not sure what to measure for. There are many factors that go into deciding which window is right for your home or business, and the measurement process isn’t always straightforward. That’s why we’ve created this short guide on how to measure windows! Read on below to get started.

Measuring size of windows

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Importance of the correct window measurements:

It’s really important to get the right measurement when you’re looking to put in a new window frame or replacement windows because an incorrect measurement can lead to a lot of problems down the line.

For one, an improperly sized window or replacement based on incorrect measurements will not fit snugly into the opening it’s supposed to go in and could end up letting in drafts and cold air. Additionally, an incorrectly sized window can actually decrease your home’s energy efficiency and increase your monthly energy bills.

Also, a badly fit window will be more difficult to open and close, which can increase the risk of something getting caught or pinched. If you’re wondering whether a window is too large or too small, it’s always better to err on the side of caution when you measure windows and go for a size that is slightly smaller rather than larger – we’ll explain how you prepare for this later on in the article.

How to measure for new and replacement windows:

The first step is to use a tape measure to find out the height and width of the opening you want to put the new or replacement windows in.

You can find the height and width measurements by measuring from the inside of one side jamb to the other if you have existing windows, this might be a little more challenging for larger or replacement bay windows so ask for a hand in measuring if you need it. Or if you don’t currently have windows you take and pencil or blue tack and place points roughly where you’d like the window to go, picturing the height, width, where the window sill will be and where the exact brickwork opening will go.

Next, make sure that your tape measure is level and you take measurements at several different points such as the left corner, centre and right corner of where you want the window to be so that you have a more accurate idea of where to place the measurements.

Afterwards, use a pencil and paper or marker board to plan out where the new or replacement windows should go. You’ll want to start by measuring from the outside edge of the window frame that’s already in place – this will give you the correct space to fit your new window if you desire height and width measurements the same as the existing frame edge.

Make sure that there is enough room on both sides of this first measurement for hinges, locks and any wooden trim around the outside of the window frame. Then measure from inside edge to find out how far back you will need your replacement or or custom order windows to go in order for them to fit the opening.

The last step is to write down these measurements and take them to your supplier or the professional you’re working with. You should have three measurements at this point, they will be both the final height, width and depth (top middle and bottom).

It’s important you get the correct measurements because with this information, they will be able to determine what size of windows you need and can move to the next step of the window installation process. It’s also worth thinking about blinds at this point as well because some installer will be able to install integral blinds at the same time as your new windows.


Should you use metric or imperial?

When you go to measure windows, whether that be for a new or replacement window, make sure to record the measurements in metric and not imperial. The reason for using metric is down to the fact you get more accurate measurements using millimetres than imperial inches.

You also want to use metric because the window manufacturer and professional window fitter you’re likely to work with will most likely be using this.

What is the 10mm rule?

If you’re wondering what the ten millimetre rule is, it’s simply a way of ensuring that your new or replacement windows are fitted snugly into the opening they go in.

This means there must be no space between either side jamb and any wooden trim around the outside of the window frame to create an airtight seal when closing. It’s also to give the installer a small fitting allowance or wiggle room when adding the new windows.

To do this you deduct 10mm from your width and height measurements. For instance, if you measure the height and it’s 870mm (87cm), you would then deduct 10mm so the exact size you provide so you can compare quotes will be 860mm (86cm).

When should you start ordering windows?

If you’re ordering custom windows, it’s best to start the process around four to six weeks before the window installation date. This will give the manufacturer enough time to make and send your new windows. If you’re using standard size replacement windows, then there is usually a quicker turnaround time of two to three weeks.

When should I get a window replacement?

If the windows in your home are draughty or letting air through, then it could be time to replace them. You can check for drafts by using a candle and placing it near the frame where you think there is an opening – if any air gets sucked into this area, that’s one sign you may need to get replacement windows.

There are other signs that your window might need to be replaced as well, these include if the windows no longer close completely or fit correctly in the frame, or if they are difficult to open and close. In most cases, it’s best to get replacement windows when the frames and sashes are still in good condition, as this will make the installation process much easier.

If you’re not sure whether or not your windows need to be replaced, get in touch with a professional window fitter who will be able to inspect your windows and give you an honest opinion.

Should you get double glazing?

The answer in most case is yes, however it depends on your personal preference, the point of the window and what you’re looking for in terms of energy efficiency. There’s a few different types of double glazing but double glazed windows on the whole do tend to be quieter, more insulated and can help keep your home warmer in the winter and cooler in the summer, so this may be something to consider if you’re wanting to make your home more energy efficient. If these are particularly important, you even consider triple glazing as well.

You’ll find that double glazing is also much easier to clean than single glazed windows, so if you’re looking for something aesthetically pleasing as well as functional then this could be the right option for you.

However, it’s worth noting that double glazing is a bit more expensive than single glazed windows, but don’t let this put you off because there is cheap double glazing, and in our option they’re a very worth while investment for a replacement window.

Final thoughts

New and replacement windows can add value to your home as well as making it more energy efficient, so if you’re thinking about getting them, make sure to do your research and get the correct measurements.

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You can save up to 65% OFF new windows and doors by using our online cost calculator.

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