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Typically, you will be deducting a further 10mm for white frames and a further 15mm for non-white frames. Please note that the extra deduction should be taken from the total width and height and not per side.

Once you have made your width and height deduction you will then have your final size to order your replacement composite door.

There are other areas to consider when measuring for replacement doors to ensure that they comply with current building regulations. These areas consist of, but are not restricted to:

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Always take measurements as viewed from the outside

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Composite door measuring
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UPVC deductions

If you are ever unsure about current legislation regarding replacement composite doors you can find further advice on our FAQ page. You can also contact us for advice or you may wish to consult a surveyor or builder. For our video guide on measuring your door, please head to our guides page.


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Always record measurements in metric (mm)

Please note, if you cannot get access to your doors safely then we recommend that you seek the help of a surveyor, builder or joiner who can carry out the measurement process for you.


​When measuring your door is it important to remember the following:

Always make sure that the final sizes include the cill and any add ons/frame extenders required

Composite Door Measuring

When taking a width measurement of your door it is important to take more than one measurement. We would recommend taking 3 separate measurements at the top, middle and bottom of the aperture. These measurements may not be exactly the same but make a note of the smallest measurement of the 3.

The same process should be done when measuring the height of the aperture by taking 3 sizes from the right, center and left hand side of the aperture from the lintel or top brickwork to the bottom of the door cill (if applicable). Again make a note of the smallest of the 3 measurements.

It is also generally best practice to check the squareness of the aperture by taking two diagonal measurements from corner to opposite corner. These measurements do not need to be recorded.

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If you have decided that you would rather buy your composite door online and organise installation instead of going through an installation company then you will at some point need to measure your existing door aperture as all replacements are bespoke to your requirements and are manufactured according to the required size of the aperture. 


​First things first, you will need a pen or pencil, a measuring tape, potentially a step-ladder and a form to note down your measurements. 

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Always measure the brick to brick opening of the aperture

Home > Guides & Help > Blog > Composite Door Measuring

Low thresholds (part M building regulations)

When you have your width and height measurement you will need to make a further reduction to compensate for the expansion and contraction of the door frame. Please refer to the table below to indicate how much the deduction should be