Strava categorizes climbs by multiplying a segment's length in meters by its grade, then sorting the result into categories ranging from Category 4 up through Hors Categorie. The Strava climb categorization is similar to the UCI's, but with some modifications. When a climb is categorized for the Tour de France, for example, there is a subjective component to the categorization. If a climb is at the end of the stage, it could get a tougher category than if it were earlier in the stage. Strava's method is objective, so if a climb is Category 1, it will always be Category 1.
Determining Climb Categories
A segment can be categorized as a climb on Strava if the length of the climb in meters multiplied by the grade of the climb is greater than 8,000. Please be aware that some segments may be categorized as climbs but not marked as such. You will see that a segment is categorized by the number on both the segment page and on the list of segments on your activity page. The categories are broken down as follows:
Cat 4 > 8000
Cat 3 > 16000
Cat 2 > 32000
Cat 1 > 64000
HC (Hors Categorie) > 80000