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 around 0.90 is a typical sweet-spot effort. IF above 1.05 is only sustainable for short efforts.

IF = NP / FTP

IF is used to categorize a ride's overall intensity relative to threshold (Coggan's reference ranges): below 0.75 is a recovery ride; 0.75–0.85 is endurance; 0.85–0.95 is tempo; 0.95–1.05 is lactate-threshold work and shorter races; above 1.05 is short time-trial or interval efforts that can't be held for an hour.

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.