When looking to replace your UPVC windows the most accurate way to measure the sizes is from outside your property.  

Although our app can help take the stress and need to take detailed measurements if you’d like to still obtain these, we recommend taking at least three measurements each for both the width and height—one from the centre and one from each side. 

Avoid measuring the old frame, as it may not reflect the true size needed.

Getting Started

Measuring your windows for replacement can depend on the style and accessibility to those windows for example upstairs windows are not easily accessible. 

Although our process once you’ve submitted designs to the app is to send a specialist to confirm measurements, this guide aims to provide clear steps on if you’d like to measure your own windows. 

If you’re replacing windows, it’s essential to ensure that the installer takes full responsibility for measuring. At Glazing Hub, we handle all window measurements for you—if there’s a problem, we’ll make it right.

For those ordering frames directly from a manufacturer, the following instructions will guide you through the process.

Before you begin, check with your manufacturer for any specific measurement requirements. While most follow standard practices, some may have unique guidelines you’ll need to follow.

Do You Measure a Window from the Inside or Outside?

As previously mentioned the best way to measure your windows including UPVC windows is to do so from the outside.

Avoid the common mistake of measuring the existing window frame—this won’t give you an accurate reading, especially if the current frame doesn’t fit properly or has moved due to damp or structural issues. 

Instead, focus on measuring the aperture opening where the window will be fitted this will give you the most accurate reading of the window size (In the UK most windows come in a standard size)

Think of it this way: once the old window is removed, you’re left with an open space—that’s the area you need to measure. 

Carefully remove any insulation or rendering to expose the brickwork edge, which forms the true boundary of the opening.

Take your measurements brick to brick for the most accurate result, if you’re unsure here please feel free to contact us and we’ll happily advise on the best approach. 

Measuring Your Windows

Step 1: 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. If you don't have a MM measurement on your tape measure you can take these down in CM and use an online convertor. 

1. Measure in metric (millimetres - MM) for the most accurate results.

2. Keep the tape measure tight to avoid slack that could skew your measurements.

3. Measure the aperture opening from brick to brick—never measure the old frame.

4. Take three measurements each for both the width and height: one from the centre and one from each side.

Useful notes

• Use the smallest of the three measurements to ensure a proper fit.

• Measure diagonally from corner to corner to check if the opening is square.

• Always confirm any specific measurement requirements with your manufacturer before ordering

Measuring the height of your windows

It’s important as we’ve mentioned previously that you take three measurements for the height to avoid any movement or slips with the tape measure

• One from the centre of the opening

• One from the left side

• One from the right side

Not all openings are perfectly square, which is why multiple measurements help identify any variations.

Always use the smallest of the three readings to ensure the new frame will fit properly, however please remember if you’re getting your windows from Glazing Hub we will always send out a professional window fitter to take more accurate measurements. 

Measure the Width of the Window

Once you have the measurements for the height now you want to repeat this process for the width of all the UPVC windows you’re looking to replace. 

• One measurement from the centre

• One from the top

• One from the bottom

Again, use the smallest reading for the most accurate fit.

Measure to Check the Frame Is Square

To determine if the aperture is square, take two diagonal measurements:

• From the bottom left corner to the top right corner

• From the bottom right corner to the top left corner

• If both measurements are equal, the opening is square.

Note: Over time, your home can shift, especially older properties which can lead to the measurements potentially becoming inaccurate. 

As a result your windows could become misaligned, a condition known as a 'racked' window. 

A significantly racked opening can complicate fitting new windows & frames, so it’s best to consult with your installer or manufacturer if misalignment is detected.

Different Window Types

As you’re measuring your existing window frames it’s also key to consider the style of windows you currently have within your home. Different window styles can have specific measurement nuances that affect accuracy and installation. 

Casement Windows

Casement windows are hinged at the side and open outward, similar to a door.

Measuring Tips:

• Measure the aperture opening from brick to brick, not including the frame.

• Take three measurements each for the height and width: one from the centre, one from the top (or left), and one from the bottom (or right).

• Since casement windows often have thicker frames, ensure that there is enough space around the opening for proper sealing.

Sash Windows

Sash windows consist of two panels (sashes) that slide vertically (or sometimes horizontally). They are common in older or period properties.

Measuring Tips:

• Measure the visible glass area and the aperture, avoiding the old frame.

• Take three measurements for both height and width from different points (top, middle, bottom for width; left, centre, right for height).

• Check for any frame warping, as older buildings may have misaligned openings due to natural shifting over time.

Bay Windows

Bay windows protrude from the building, creating a small shelf or nook inside the home. They usually consist of three or more panels.

Measuring Tips:

• Measure each window section individually—typically, the central pane is larger than the side panels.

• Measure both the width and height from the centre and both sides of each section.

• Take additional measurements for the angles between the panes to ensure an accurate frame fit.

• Double-check measurements for each section to account for the bay’s unique shape and protrusion.

Tools to measure your windows

When you’re looking to measure the size of your window frame the below are the ideal tools you’ll need to ensure you take accurate measurements:

• Measuring tape (preferably metric)

• Spirit level (to check frame alignment)

• Notepad or smartphone (to record measurements)

• Screwdriver or utility knife (to remove obstructions)

• Safety gloves and glasses (for protection)

• A strong brew

Window Measurement FAQs

• Can I measure my windows from the inside?

  • While you can take measurements from the inside, the most accurate results come from measuring the outside of the building. This ensures you capture the true size of the aperture, including any external brickwork or structural elements that may affect the frame size.

• What happens if my window opening isn’t square?

  • If the opening isn’t square, meaning the diagonal measurements don’t match you’ll need to use the smallest width and height measurements for your order. Significant misalignment (known as racking) may require professional advice, as it can affect how well the new frame fits and functions.

• How do I measure for replacement windows in an older property?

  • Older buildings often have irregular openings due to natural settling. Measure from brick to brick, taking three readings for both the width and height. Double-check for squareness and consult with your installer to assess any additional adjustments required for uneven frames or historical features.

• How accurate do my measurements need to be?

  • Accuracy is crucial. Measure to the nearest millimetre (mm) and always use the smallest measurement from your readings. Even a small error can lead to fitting issues or gaps that compromise energy efficiency.

• Can I install a new frame without removing the old one?

  • In some cases, especially with older properties, new frames can be fitted over existing ones. However, this is generally not recommended, as it can reduce window size, affect insulation, and hide underlying structural issues. A full frame removal typically offers the best results.

• How much space should be left for sealant during installation?

  • Most manufacturers recommend leaving a tolerance gap of 5-10mm around the window frame to allow for installation and sealing. Too little space can make fitting difficult, while too much can lead to gaps, reducing insulation and energy efficiency.

• Do different window materials require different measurements?

  • The measuring process remains consistent across materials—whether you’re fitting uPVC, aluminium, or timber. However, the thickness of the frame can vary by material, so always confirm specific installation requirements with your manufacturer or installer.

• What are the consequences of incorrect measurements?

  • incorrect measurements can lead to several issues, such your new windows not fitting within the aperture or worse your new window frames having gaps leading to reduced energy efficiency and increased drafts