Category 4: Usually less than 2 miles in length. Climbs of 250 to 500 feet in elevation gain.
Category 3: Between 2 and 3 miles in length. Climbs of 500 to 1,500 feet in elevation gain.
Category 2: Between 3 and 6 miles in length. Climbs of 1,500 to 3,000 feet in elevation gain.
Category 1: Between 6 and 12 miles in length. Climbs of 3,000 to 5,000 feet in elevation gain.
Hors Catégorie (HC) or Above/Beyond Category: Usually more than 12 miles in length or with grades often exceeding 10 percent. Climbs of 5,000+ feet in elevation gain.

[24 Percent Grade]
At TourOfCalifornia.org, rides between 2006 and 2008 were recorded using a CicloSport HAC 4 cycling computer, which is known for elevation gain values on the low end. More recent rides were logged using a Garmin Edge 305 or 500.
See also Top Bike Climbs in California and Top Bike Climbs in the United States.