Imperial sizes are the traditional sizes measured in "inches" or "feet and inches" Metric sizes have been used by builders of new homes for about 30 years, they are a different height and have different widths/thickness from the imperial sizes.
Imperial sizes are normally 1.3/8th inches thick (35mm) for older doors, the two larger sizes may be 40mm or 45mm.
Metric doors are normally 40mm thick (1.5/8th) for more modern doors.
All FD30 (1/2 Hour fire doors) are 1.3/4” inches - 45mm thick.
Bellow see a table for different door sizes that are available, as not all doors are available in all sizes. (The PDF at the end of the page is easy to print guide.)
Please feel free to print the following pdf for reference, conversions are as accurate as possible.