How to Measure Windows for Replacement – Help Guide
Replacing your windows can improve energy efficiency, reduce draughts, and modernise your home.
But before ordering new windows, it’s important to measure the existing openings correctly.
This guide explains how to measure windows for replacement step-by-step so you can get accurate sizes before requesting a quote or ordering new frames.
What You’ll Need
Before you start measuring, make sure you have the following basic items and information where applicable:
- Spirit level
- Tape measure
- Pen and paper
- Step or access ladder
- New considered frame details
- Door threshold details
- Cill requirements
- External finish review
- Internal finish review
- Ventilation requirements
Always measure in metric (millimetres not centimetres) – most window suppliers and installers in the UK work in millimetres (mm)
Step 1: Check the Quality of the Opening
Before taking any measurements make some basic checks first:
Use a spirit level to check:
- Sides (plumb)
- Top and bottom (level)
Also, check for perimeter straightness and edge irregularities as the odd stone or brick may be protruding into the opening.
Additionally, a diagonal corner to corner measurement check will indicate if the opening is square.
Finally, check for any beam deflection at the top which can sometimes be present for large spans.
NOTE: If there are any quality issues with the opening you need to make a judgement to establish a frame fitting allowance. This is to ensure that the new window or door fits cleanly into the opening and can be independently and correctly positioned without binding or catching on any sides of the aperture – STEP 2 explains how to do this.
How to Measure Windows for Replacement
Step 2: Measure the Width of the Opening
Take a measurement of the aperture width in three places:
- Top
- Middle
- Bottom
Use the smallest measurement as your final aperture width
EXAMPLE:
- Top: 1205 mm
- Middle: 1203 mm
- Bottom: 1207 mm
Final aperture width = 1203 mm
Next, make a small deduction to the final aperture width – this is the fitting allowance and usually 5mm per edge.
With a fitting allowance of 5 mm per edge this is therefore a 10mm deduction from the final aperture width – hence 1203 minus 10 = 1193
Therefore, the frame width for manufacturing is 1193 mm
Step 3: Measure the Height of the Opening
Take a measurement of the aperture height in three places:
- Left
- Centre
- Right
Use the smallest measurement as your final aperture width
EXAMPLE:
- Left: 1102 mm
- Centre: 1100 mm
- Right: 1103 mm How to Measure Windows for Replacement
Final aperture height = 1100 mm
Next, make a small deduction to the final aperture height – this is the fitting allowance and usually 5mm per edge
With a fitting allowance of 5 mm per edge this is therefore a 10mm deduction from the final aperture height – hence 1100 minus 10 = 1090
Therefore, the frame height for manufacturing is 1090 mm
Step 4: Decide the cill size
If you require a projection cill you need to check where the position of a new frame is going to be fitted within the reveal i.e. the ‘in and out position’.
Once you have established the frame position you can then select the correct cill size to provide an adequate overhang to the outside wall
We offer a range of projection cill sizes to meet your installation requirements – please contact us directly if you require an alternative solution to our projection cill range.
Measuring the Outside AND Inside
Measurements for the final frame size should be taken from the OUTSIDE – this allows for a more accurate visual assessment of the aperture that the window or door will be fitted into.
However, measurements should also be taken from the inside to compare with the outside – this is to understand the impact of any internal finishes on the new frame.
Consideration should be given to excessive frame cover on the inside due to plaster reveals, tiling or window boards, etc. Unless you are remodelling the entire inside aperture, you may need to use an alternative window or door system with a wider profile or use add-ons to overcome internal finish issues.
Don’t forget external finishes as well, e.g. render or cladding, as this will also have a cover impact on the new frame to the outside.
Please contact us directly to discuss options for overcoming internal and external finishes.
Common Window Measurement Mistakes
Avoid these common mistakes when measuring:
Condition of basic tools
Make sure that measuring tools and spirit levels are working correctly.
Only taking one width and height dimension
Always measure in at least three places to find the tightest point.
Using centimetres instead of millimetres
This can cause accuracy errors when ordering – only use millimetres.
Not allowing a fitting tolerance
Prevents the window or door from going into the aperture cleanly.
Ignoring uneven openings
Edges that are not straight can create large gaps and unsightly sealing. How to Measure Windows for Replacement
I’d wholeheartedly recommend Marlin Windows. We ordered Steel Replacement French Patio doors and Windows. They are absolutely beautiful. And they were an absolute pleasure to deal with. Thanks Marlin!
Myar Craig-Brown – London
Need Help Measuring Your Windows?
If you’re planning to replace your windows and want expert advice, our team can help.
How to Measure Windows for Replacement
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Frequently Asked Questions
If you require any support, please don’t hesitate to review our FAQ’s below or drop us a message on our contact form here
How slim is the Heritage frame?
The Alitherm Heritage outerframe has a sight line of 33mm.
The Classic Heritage outerframe has a sight line of 35.5mm.
Is the Heritage system suitable for listed buildings?
The Heritage slim systems have already been approved for listed building projects by many councils including:
London Boroughs of Islington, Camden and Tower Hamlets, Watford Borough Council, Essex County Council and Leeds City Council as well as Cadw, the Welsh Government’s historic environment service.
What thermal rating does the Heritage system achieve?
Window Energy Rating ‘B’ – U Value 1.4 W/m2K (using sealed unit 0.9 W/m2K)
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