These guidelines are to help you measure for a new double glazed door and/or window and make an accurate measurement.
Measure the opening outside on both the height and width. Always measure the exact brickwork opening and not the old frame or door.
Measure brick to brick in at least three different places and deduct 10mm from both the final height and width measurements to allow for 5mm fitting allowance around the new frame. This will enable you to ensure the new frame sits plumb and square once in the aperture. Spacers and filler will then be required to secure the frame into place and fill any gaps.