How to Measure Windows and Doors Guides | Windows plus Doors

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Before you begin, you will need:

  • Tape measure
  • Pencil and paper

Important notes to bear in mind:

  • Always record measurements in millimetres as this will give an accurate reading
  • Measure the window or door space externally
  • Take measurements from brick to brick, not the old frame
  • Measure in three different places and take smallest as final measurement
  • Never measure the old frame
  • Check, and check again

These guidelines are simple to follow. However, if you’re unsure, we recommend consulting a professional fitter or joiner to help you measure up before you place your order.

How to measure external doors


To measure the width of the door, it’s important to take at least three measurements. One at the top, middle and bottom. Use the smallest of these as the width and it’s important to deduct 10mm from the final measurement to ensure there’s room to fix into the gap.


The height should also be measured in three different places, from brick to brick. Left, middle and right of the opening. Again, always deduct 10mm from the smallest measurement, this is your final height.

With side panels or top light

Follow the steps above to measure your door. To measure the side panel, you should begin from the centre point of the door frame to the edge of the brickwork. Again, deduct 10mm from the total width of the door and side panel opening.

How to measure for new windows


Measure from brick to brick in three different places – top, middle and bottom to ensure an accurate measurement. Take the smallest measurement as the final width and deduct 10mm for fitting space.


Again, the height should be measured from brick to brick at three points – left, middle and right. It is important not to measure from the existing frame as your figure won’t be correct. Deduct 10mm from the final height.