1. Measure the diagonals from corner to corner to check opening is square.
The two measurements need to be the same.
2. Measure the width of the opening in three places, top, middle and bottom.
Use the smallest width measurement.
3. Measure the height in three places, left, right and the middle.
Use the lowest height measurement.
PLEASE NOTE: We will allow a 10mm tolerance from the structural opening dimensions you provide. If you are going to make the threshold flush with your inside floor level, then you need to sink part of the frame below the finished floor level. This amount differs depending on which door you are having and which type of threshold you have chosen. We will confirm this with you once your order is received or give us a call so we can talk you though it.