Intensity Factor (IF)

IF is a ride's Normalized Power divided by your FTP. It's a dimensionless 'how hard was this ride relative to my threshold?' number. IF 1.00 is an hour at FTP. IF 0.80 is a typical sweet-spot ride. IF above 1.05 is only sustainable for short efforts.

IF = NP / FTP

IF is used to categorize the physiological character of a ride: IF ≤ 0.65 is recovery; 0.65–0.80 is endurance; 0.80–0.90 is tempo; 0.90–1.05 is threshold work; above 1.05 means intervals.

IF is also the intensity factor in the TSS formula — it's how 'how hard' is encoded into a single number when computing training stress. A 60-minute ride at IF 0.70 generates 49 TSS; at IF 0.85 it generates 72 TSS.