To measure your structual opening firstly ensure there is no pre-existing frame installed.
Next, take 3 measurements for the width, one at the top, middle and bottom of the opening. Use the smallest of these measurements. Then, take 3 measurements for the height, one on the left side, middle, using the smallest measurements for that too.
Your structural opening will then be the smallest width x smallest height.
We also have a video showing how to measure your opening:
All of our steel doors are supplied pre-hung in their frame. For the strongest installation we advise fixing our steel frames directly into the aperture.