Training Stress Score (TSS) Calculator

Training Stress Score is a single number that quantifies how much training load one session puts on your body, combining how long you rode and how hard you rode relative to your fitness. It was defined by Dr. Andrew Coggan and is used widely in cycling and triathlon. By definition, one hour ridden at exactly your Functional Threshold Power equals 100 TSS, so a 200 TSS ride is roughly twice that stress.

TSS uses the formula (seconds times Normalized Power times Intensity Factor) divided by (FTP times 3600), then multiplied by 100. Here seconds is the session duration, Normalized Power is the smoothed power your device reports, FTP is the most power you can hold for about an hour, and 3600 is the number of seconds in one hour. Because Intensity Factor equals Normalized Power divided by FTP, the result also equals the Intensity Factor squared times the hours ridden times 100.

Intensity Factor (IF) is your Normalized Power for the session divided by your FTP, so it describes how hard the ride was relative to your threshold. An IF near 1.0 means you rode close to threshold, while easy recovery rides sit below 0.75 and short, very hard efforts climb above 1.15. This calculator computes the Intensity Factor for you from the Normalized Power and FTP you enter.

As a rough guide from TrainingPeaks, under 150 TSS is a low load where you usually recover by the next day, 150 to 300 leaves some fatigue the following day, 300 to 450 is a hard day that can take about two days to clear, and over 450 is an epic effort that may linger for several days. These bands are general guidance, since recovery depends on your fitness, sleep, and nutrition.

Normalized Power is an adjusted average that weights the harder surges of a ride more heavily than coasting, giving a better picture of physiological cost than a plain average. Your bike computer, power meter app, or platforms like TrainingPeaks, Strava, or Garmin Connect report it for each ride. Enter that reported Normalized Power here rather than the simple average power.

This calculator is built for power-based cycling and triathlon bike sessions, where Normalized Power and FTP are measured in watts. Running and swimming use their own stress scores (rTSS and sTSS) based on pace or speed rather than watts, so the numbers here will not match those sports. Use it for rides where you have a power meter and a known FTP.