If you are planning to fit the product into the reveal (recess, architrave) of the window – you need to measure the width and height 3 times at different intervals. A good rule of thumb is to measure ‘top, middle & bottom’ and ‘left, centre & right’.
Once you have collated the three measurements, you need to take the smallest measurement – we would then deduct an additional 5mm or 10mm off to allow for tolerance.