Inhouse product
Windows are usually measured from the brickwork opening from the outside of the property, always in millimetres (mm).
Firstly, measure the width, brickwork to brickwork, in three places (top, middle and bottom) and deduct approximately 10mm from your smallest measurement.
Secondly, measure the height of the opening in a few different positions and again deduct approximately 10mm from the smallest measurement. In case of a metal lintel please exclude it. Most windows sit on a cill, if so then please measure from underneath the existing cill unless it's not removable (e.g. stone).