Windows don’t last forever – years of weather exposure, UV radiation, heat, cold, condensation & general ageing can see window frames / units degrade until the point that they are no longer efficient, no longer look as good as they once did and become, overall, visually less appearing on your home.

New windows can improve a properties thermal efficiency, help to reduce energy costs and give a house an updated look. However, when it comes to replacing the windows in your home, measuring them accurately is key to ensure they fit properly. Here we look at key things needed when measuring replacement windows and what needs to be considered when making this decision.



Why Should You Replace Your Windows?

There are several reasons why you might consider replacing your windows. Some of the most common reasons include:

Energy Efficiency

Old and worn-out windows can allow air to escape from your home, causing your energy bills to skyrocket. Replacing your windows with more energy-efficient options can help you save money on your monthly energy bills. With energy bills in the UK at the highest they’ve ever been – there has never been a more important time to improve your homes thermal efficiency – reducing the need for heating and subsequently saving you money.

Increased Security

Older windows can be easier to break into, making your home more vulnerable to burglaries and break-ins. Replacing your windows with more secure options can give you peace of mind and protect your home and family.  In the UK, windows must meet certain security standards to ensure that they provide adequate protection against break-ins and burglaries. These standards are set by the British Standards Institution (BSI) and are commonly referred to as the BS 7950 and PAS 24 standards.

Improved Appearance

Replacing your old and worn-out windows can significantly improve your home’s curb appeal and overall appearance.


Service man measuring window for installation indoors, closeup


Measuring your windows to get a quotation for replacement units:


What Points To Measure From?

When considering the installation of replacement windows, it is important to accurately measure existing window frames. Measurements should be taken from a variety of points on the frame in order to ensure proper fit for the new window. These points include the width and height at each side of the frame; the distance between bottom corner and top corner diagonally; any obstructions within or outside the frame such as moldings around glass, air conditioners, shutters etc.; and finally, any additional requirements such as sills or trimming that may need to be cut away.

It is also important to note whether measurements are taking from inside or outside edges of the frame. This will help determine which type of window needs to be installed since some require an exact outer dimension while others may require measurements taken from inner dimensions. Additionally, if there are multiple levels involved with a particular installation, each level must have its own individual measurements taken so that all necessary components can properly fit together.

In addition to measuring existing frames, other factors must also be considered when installing replacement windows such as climate conditions and energy efficiency ratings. By researching local building codes prior to purchasing new windows, homeowners can rest assured they have chosen one that meets their specific needs while adhering to local regulations. Taking time to make sure accurate measurements are taken from various points on an existing window frame will ultimately result in a successful installation experience for all parties involved.


What Dimensions Will You Need?

When measuring guides for replacement windows, it is important to know what dimensions will be necessary. The width and height of the window must first be established before considering other measurements. In some cases, an exact fit may not be possible due to variances in wall construction or opening size.

The next step is to determine how far out from the wall the new window should extend. This measurement will depend on whether a drywall return or jamb extension are needed to cover any gaps between the frame and wall surface. If so, this additional depth should also be factored into the overall length of the window unit.

Further details such as sill depths and head heights can then be taken into account depending on individual preference. It is recommended that all measurements are double-checked prior to ordering a custom-made window system; accuracy at this stage is essential for achieving a successful installation outcome.


Measuring Edge To Edge On The Frame

Measuring edge to edge on the frame is a key step in determining the correct size of replacement window. Taking these measurements requires certain tools, techniques and knowledge that should be used properly for accurate results. It is important to measure from inside the existing frame instead of outside it.

In order to take an exact measurement, one must use a ruler or measuring tape with markings that are at least 1/8th inch apart. The width is measured first by placing the ruler or tape at the top of the inner frame vertically and then extending it across until it reaches the other side of the same frame horizontally. The length follows similar steps except it is measured from top left corner diagonally down to bottom right corner.

Once all measurements have been taken, they can be compared against standard sizes available for replacement windows so that a suitable choice can be made according to individual needs. Before any purchase takes place however, double-checking accuracy of measurements is highly recommended as errors can lead to incorrect installation or even damage if wrong sized parts are used.


Standard External Windows Sizes In The UK

Standard external window sizes in the UK vary according to the style of window chosen. The most common size for a single window frame is 900mm x 1125mm, though this can be altered depending on the particular needs of the customer. For double windows, it is typically 1800mm x 1125mm. Some types of frames may also require different measurements due to their design and structure.

The importance of measuring accurately when replacing existing windows cannot be overstated; incorrect measurements can result in poor insulation or even structural damage if not installed correctly. Therefore, it is essential that accurate figures are taken prior to purchasing new replacement windows in order to ensure they fit perfectly into an existing space.

Measuring should include edge-to-edge measurements on both sides of any given frame so as to determine any irregularities which might occur during installation process. A qualified fitter would need these details before being able to install the windows safely and effectively without compromising either safety or aesthetics.




How do you measure for replacing windows?

  1. Measure the width: Measure the width of the window frame from one side to the other, taking measurements at the top, middle, and bottom of the frame. Record the smallest measurement.
  2. Measure the height: Measure the height of the window frame from the top to the bottom, taking measurements at the left, center, and right of the frame. Record the smallest measurement.
  3. Measure the depth: Measure the depth of the window frame from the inside to the outside, taking measurements at the top, middle, and bottom of the frame.


Do replacement windows need to be exact size?

Not exactly – replacement windows and the frame can vary from your current window frame size – there is usually a tolerance of around 1/4 of an inch – as window frame units are screwed into the brick – a gap needs to be available for slightly oversized units.

In This Article…


Laura Wakeham

Laura is an experienced writer with a background in the construction, wood work and hardwood industry - having written for various nationwide publications including Houzz, Construction Weekly and other more local editorial platforms. With over 10 years experience, Laura has detailed knowledge on the ins and outs of hardwood fabrications from orangeries to conservatories, windows and doors.

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