About the picture above with the hardness of the stages:

The way the numbers are calculated is to take the sum of the square of the average percentage for each climbing kilometre during a stage (p_1^2+p_2^2+p_3^2+..., where p_i denotes the average percentage of the ith climbing kilometre of a stage). It seems like a good way of measuring, because very steep parts are taken into consideration properly.

Thomas De Gendt asked the uploader on Twitter to also make a list of the easiest stages. The uploader cheekily answered that the Mont Ventoux stage of 2016 must be the easiest of all time, since Froome didn't even need his bike to get through the stage (that De Gendt incidentally won).

De Gendt drily countered that at least he had won the hardest GT stage of the century, which turned out to be true. The Stelvio stage of 2012 scored no less than 4332 points on this scale. The Ponte di Legno stage of next year is the 5th hardest Giro stage of the century.

https://twitter.com/cycling_visual/status/1078004884484562945