1. Width – Measure at the top on the inside of window from left to right. Measure to the closest millimetre. Deduct 7mm from the measurement taken at the top of the window and record this measurement as the Width.
2. Height - Measure at three different locations on the inside of window from top of the reveal to the window sill. Measure to the closest millimetre. Use the smallest height and deduct 10mm and record this measurement as the Height.