Your ABILITY is defined by your weight in kilograms and your Peak20min power in watts. These two parameters define how fast you will be, and what category you should choose to ride with. The lower chart displays three ability categories with two curves that represents the borders between these three categories. These two curves, are ISO-SPEED curves, that tell us what relative power in watts/kg riders of different weights need to achieve EQUAL speed at 3% inlcline with “hands on hudz” riding position.
If you are not sure where to find your 20min peak power you can connect your Strava account to Intervals.icu.  Click Best Efforts tab on top right corner and enter 20min into the field.

To calculate your racing/riding category enter your Peak20min ability in watts and weight in kilograms on the table below:

We could use ISO-speed curves at pan cake flat or 0% incline with hands on drops and get very steep curve, which would favor (give a lot of advantage) to lighter riders. We could also use ISO-speed curve at steep 10% incline with hands on tops and get very shallow (almost linear) curve, which would favor (give a lot of advantage to heavier riders). Thus, we decided that 3% incline is somewhat sweetspot of compromise between lighter and heavier riders, with first having slight advantage on steep slopes and later having slight advantage on flats and downhills.