Climbs - how is the CAT calculated?

Originally Posted by ed073

What do the climb categories mean?
Climbs are graded according to their severity, from fourth to first category, plus the very hardest climbs which are considered 'hors categorie' or 'beyond category'. The gradings take into account the length and steepness of the climb and the position of the climb in the stage, with the quality of the road surface making up a final, less important factor.

How are the gradients calculated?
A grade of, say, ten percent, simply means that the road ascends ten metres for every 100m it travels horizontally. For those with a mathematical bent, it's a simple tangent ratio. Climb grades are averages for the whole climb, so a straightforward-sounding six percent grade can hide some much steeper pitches that make it a monster.

I know that climbs are categorised Cat 1, 2, 3, etc., but is a "1" more difficult than a "4", or is it the reverse?

The categorisation of climbs is determined by the race organisers, and is based on the steepness of the climb, the position of the climb on the stage, and to a lesser extent, the quality of the road surface. Generally speaking, a climb needs to have an average gradient of around 4-5% and a vertical elevation of at least 100 metres for a climb to be categorised. A hors categorie (literally meaning 'out of category') climb is the most difficult, with an elevation usually 1,500 metres (5,000 feet) or more above sea level - while a category 3 climb is one of the easiest, with an elevation usually in the range of 300-600 metres (1,000-2,000 feet) above sea level.

Originally Posted by Eldrack

Not entirely, and tbh I don't think its an exact science, hence the lack of replies. Catagory depends on 4 things (I think), length, steepness, position in the race, quality of the road (width, surface etc). Not entirely sure about how much that last one makes a difference though.

Take the Cauberg for example, last climb in stage 3 this year.
http://www.salite.ch/cauberg.htm. Its not very long, but has a steep section in the middle. Probably looking at a cat 4. However as it is right near the end of the stage everyone is going to be going up it real fast, hence the third catagory tag.

If you want to catagorise climbs at home, try comparing them to climbs in the TdF. Climbs of similar length/gradient. Won't be exact but you should get some idea.