The Strava climb categorization is similar to how the UCI categorizes climbs 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 was earlier in the stage. Strava’s method is objective, so if a climb is a Category 1 climb it will always be a Category 1. You will see that a segment is categorized by the number on both the segment page and at the list of segments on your activity page.

To decide the category of a climb Strava multiplies the length of the climb (in meters) with the grade of the climb. If that number is greater than 8,000 then it is a categorized climb.

CAT 4 > 8,000
CAT 3 > 16,000
CAT 2 > 32,000
CAT 1 > 64,000
HC > 80,000

HC – “HORS CATEGORIE” (a French term for the above category) climbs are the hardest rating/score given to any climb. All climb scores are based on distance, grade/elevation change, and maximum elevation. The combination of these factors drives all final climb categories and there is no subjective analysis used in the final scoring of any climb score. HC climbs will traditionally be very long (over 10 miles), very steep (average grades above 8 to 10%), or very high (above 11,000 feet) but again some extremely steep or long climbs could alone qualify it as an HC rated climb.