How do I measure a window opening correctly?

Measure the structural opening (brickwork to brickwork), not the existing frame. Take the width in three places — top, middle and bottom — and the height in three places — left, middle and right. Always work to the smallest measurement in each direction, because openings are rarely perfectly square. Check the diagonals too: if they differ by more than a few millimetres, the opening is out of square and you’ll need to allow for it. Record everything in millimetres to avoid conversion errors. This is the aperture, you will need to deduct off a tolerance gap for the manufactured window, usually around 10mm (5mm each side). Please see our full SevenDay how to guide on measuring for a new window.