How to Measure a Window (for Replacement)

Measuring a window for replacement is a straightforward job. However, if you don’t get your measurements right, then you’ve got problems.

Get the measurements wrong by only a few centimetres and your new frames might not fit.

For anyone replacing windows, you should insist that the responsibility for measuring is taken by the installer so they are liable for any issues.

A reputable manufacturer and installer will measure the windows for you, so make sure you discuss and confirm this before going ahead. At Everest, we are responsible for all measurements of windows, so if there is a problem, we put it right.

If you want to measure your windows as a rough guide for getting quotes then follow the instructions below. But, make sure the measurements are taken again by the installer.

Anyone that wants to order frames directly from a manufacturer can use the instructions below as a guide.

First of all, check with your manufacturer to see if they have any specific requirements. Most manufacturers measure windows in the same way, but some do have differences.

Do You Measure a Window from the Inside or Outside?

The most accurate measurement for a window is taken on the outside of the building.

Don’t make the mistake of measuring the window frame! The measurements should be taken of the aperture opening where the window is fitted. Otherwise, you could just be measuring an existing badly fitting frame.

If you imagine that once an old window is removed from the wall leaving a hole, that is the area you want to measure. Try to scrape away insulation and rendering to get to the edge of the brick that will be the edge of the opening.

Measure brick to brick.

Hold the Tape Measure Straight and Taut

To get a really accurate measure, the tape measure must be held tight. For a window that is bigger than you can stretch, it’s better to use two people.

Keep the tape measure straight from point to point across the frame so you are not measuring on a diagonal.

Take your readings in metric millimetres as this is the standard used.

Measure the Height of the Window

Take three measurements for the height:

• One from the centre
• One from each side of the window opening

The reason we take more than one measurement is to get the average reading as not all frames are fully square.

Once you have your three readings, the smallest measure would be the one you use.

Measure the Width of the Window

Repeat the process for measuring the height and take three readings for the width. One in the centre and one on each side.

Use the smallest reading.

Measure to Check the Frame Is Square

Check to see if the aperture is square by taking a diagonal measurement.

Take two measurements from the bottom left to the top right and then the bottom right to the top left.

If the measurements are the same, your window opening is square.

It’s not uncommon for window frames and openings to drop and become misaligned over time as buildings naturally shift. You can see this clearly on older properties.

This is known as a 'racked' window and a window that is racked significantly can cause problems fitting new frames. Discuss this with your installer and manufacturer.

The 10mm Deduction Rule

Some manufacturers will ask to deduct between 5-10mm from the brick-to-brick measurements.

When a window is installed, the frame needs some tolerance around it to allow for it to be installed and to accommodate natural shifting.

• Too little tolerance and the window might not fit after the sealant is applied.
• Too much tolerance and the window will have micro gaps between the frame and wall reducing energy efficiency.

Ask your manufacturer how much they want to deduct from the measurements for tolerance.

Don't Forget the Old Rule 'Measure Twice; Cut Once'

Check your measurements at least two or three times to make sure you have them right. It’s much less costly to take the time when measuring than to find you have a problem after frames have been made.

Be clear when giving your measurements to a manufacturer what you have measured.

All advice given is a guide only, so please check with your installer or manufacturer about how they specifically want their measurements taken.