As you can see, these definitions are subjective. The origin of these categories is a bit of a mystery, but I heard a story that long ago (early part of last century) the categories arose from the gear required to get up the hill in a car. The early cars didn't have much power, so steep climbs required shifting down to 2nd or 1st gear, some even required going in reverse (the smallest gear ratio in cars). The climb's category then corresponded to the gear required to get the car up it. That's how the fable goes, anyway.
- "The easiest is a Category 4, which is typically less than 2km long and about 5 percent grade, or up to 5km at a 2-3 percent grade.
- A Category 3 can be as short as one mile with a very steep grade, perhaps 10 percent; or as long as six miles with a grade less than 5 percent.
- A Category 2 can be as short as 5km at 8 percent, or as long as 15km at 4 percent
- A Category 1, once the highest category, can be anything from 8km at 8 percent to 20km at 5 percent.
- An hors catégorie (“above category”) rating is given to exceptionally tough climbs. This could either be a Category 1 whose summit is also the finish of the stage, or one that is more than 10km long with an average grade of at least 7.5 percent, or up to 25km miles long at 6 percent or steeper."
Regardless of lore, categorization of climbs does not adhere to a strict formula, so a category 1 in France does not necessarily equal a category 1 in the US. Roads in the US tend to be newer and try to maintain lower grades, so you rarely see the grade and length of climbs here that you can find in Europe.
Speaking of grades, ever wondered exactly how they calculate those 7% climbs in Le Tour? From VeloNews' live stage 10 coverage [link now bad] of the 2008 Tour de France comes a very simple definition of a climb's percentage:
"If a road rises 10 meters over a 100 meter length, it is a 10% slope. That means that a 100% slope would equal a 45 angle, rising exactly what it travels. Most interstate highways in the U.S. are built with the goal of not exceeding a 5-percentIf you need more details about the measurement of climbs, stop by everybody's favorite reference, Wikipedia.
slope. There are plenty of exceptions, of course, but that was the original goal."
[Links in this post updated 7/12/11]