TSS (Training Stress Score)
A single number estimating how hard one workout was, combining duration and intensity into one score.
Training Stress Score (TSS) is a single number that estimates how much a workout took out of you, combining how long it lasted with how hard it was. It was developed by Dr. Andrew Coggan as part of the Performance Management Chart model that TrainingPeaks popularized, and it's now the closest thing endurance sport has to a common currency for "how much training did I actually do."
How it's calculated
For cycling, the original formula uses Normalized Power (a power average that weights hard surges more heavily than steady efforts) and Functional Threshold Power (FTP): an hour ridden at exactly your FTP scores exactly 100 TSS, by definition. A two-hour ride at 70% of FTP scores less than 100 despite taking twice as long, because intensity matters more than duration in the formula. Running and swimming have their own pace-based or heart-rate-based variants (often written rTSS, sTSS, hrTSS) since power meters aren't universal in those disciplines.
What it doesn't measure
TSS scores the session, not your fitness. A single 150-TSS ride doesn't tell you whether that was a huge effort for a beginner or a moderate day for someone deep into an Ironman build — that context comes from how load accumulates over weeks, which is what Chronic Training Load (CTL) tracks. Comparing raw TSS numbers between two different athletes is close to meaningless; tracking your own TSS over time, against your own history, is where it becomes useful.
Using it in a plan
TSS is the raw input a periodized plan is built from — it's what gets summed and averaged into the weekly and rolling-average numbers that actually drive decisions about when to push volume and when to back off. On its own, a single day's TSS is just a number; what matters is the shape it makes over weeks, which is why it feeds directly into concepts like CTL and ATL rather than being read in isolation.