To decide the category of a climb we multiply the length of the climb (in meters) with the grade of the climb in percent. If that number is greater than 8000 then it is a categorized climb. The minimum percent grade must be 3% or higher. So, for a 4% avg grade climb, it has to be 2km at least to be categorized. Cat 3 = 16000, 2 = 32000, 1 = 64000, HC > 80000.

I believe that this equates in strava terms to

Cat 4 80m climb

Cat 3 160m climb

Cat 2 320m climb

Cat 1 640m climb

HC 800m climb

all must be min 3% gradient.