What is Training Stress Score?
Training Stress Score (TSS) quantifies the total training load of a single workout as a single number. The formula is: TSS = (NP / FTP) ² × duration in hours × 100, where NP is Normalized Power and FTP is Functional Threshold Power. An equivalent form uses Intensity Factor: TSS = IF² × hours × 100.
A one-hour ride at exactly FTP produces a TSS of 100. In practice, different TSS values correspond to different levels of effort: under 150 is typically an easy to moderate ride, 150 to 300 is a hard training day, 300 to 450 is a very demanding effort like a long race, and above 450 represents an epic effort that may require multiple days of recovery.
TSS is the input to the Performance Management Chart. Daily TSS values feed into Chronic Training Load (CTL) and Acute Training Load (ATL), which together drive training decisions around volume, intensity, rest, and race readiness.