Hi Marcos - I don't know how grand tour Climb Categories are worked out, but these are the rules that we use...
To work out the Category of a climb, we multiply the length of the climb in metres by the gradient of the climb in percent to determine a score. This score is then used to determine the Category as follows:
Category 4 | 8000 or above |
Category 3 | 16000 or above |
Category 2 | 32000 or above |
Category 1 | 64000 or above |
Hors Catégorie | 80000 or above |
For example, a climb that goes on for 1km at an average gradient of 12.5% has a score of 1000 x 12.5 = 12500, which equates to Category 4.
I'm not sure it the article from Bicycling of September 2014 is acurate but it states:
According to European cycling tradition, climbs fall into four categories based on their difficculty; a fifth, hors (above category),
is reserved for the most difficult routes. Here'S the breakdown CAT 4 ' "These easier Climbs have average grades of 2 to 3 percent over 3 miles, or up
to 5 percent for climbs shorter than a mile CAT 3 Climbs with rates that average less than 5 percent over 6 miles or much shorter ascents up to
10 percent CAT 2 Ascents up to 4 percent over 9 miles or with an 8-percent grade over 3 miles CAT 1 Climbs up to 12 miles long at 5 percent or 5-mile
pitches at up 8 percent HORS CATÉGORIE Average grades of 7 percent over at least 6 miles or 6-percent grades that span 15 miles or more—C.J.
Regards,