What Is TSS? Training Stress Score Explained
Training Stress Score (TSS) explained — how it combines duration and intensity into one training-load number, how it is calculated from FTP and normalized power, what the numbers mean, and how it drives fitness and form.
Training Stress Score, or TSS, rolls the two things that make a session hard — how long it was and how intense it was — into a single number. It lets you compare a brutal one-hour interval session with a steady four-hour ride on the same scale, and it is the raw material for tracking fitness and fatigue over a season.
How TSS is calculated
The scale is anchored to your functional threshold power (FTP): one hour ridden exactly at threshold is defined as 100 TSS. Everything else is measured against that. The power-based formula is:
TSS = (seconds × NP × IF) ÷ (FTP × 3600) × 100
where NP is normalized power (a fatigue-weighted average that punishes surging) and IF is the intensity factor, NP ÷ FTP. In plain terms: ride harder relative to threshold and TSS climbs with the square of intensity; ride longer and it climbs in proportion to time. The TSS calculator does the arithmetic from your duration, normalized power, and FTP.
What the numbers mean
A single session's TSS gives a quick read on how much it will take out of you and how long you may need to recover:
| Session TSS | Rough recovery demand |
|---|---|
| Under 150 | Fresh again by the next day |
| 150–300 | Some residual fatigue into the next day |
| 300–450 | Lingering fatigue for a couple of days |
| 450+ | A big day — expect several days to absorb it |
From single sessions to fitness and form
The real power of TSS is what happens when you track it over weeks. Feeding daily TSS into an impulse–response model produces three trends: CTL (chronic training load, a ~6-week average — your fitness), ATL (acute load, a ~1-week average — your fatigue), and TSB (CTL minus ATL — your form, or freshness). Building CTL steadily while managing ATL is the whole game of periodisation, and arriving at a race with slightly positive TSB is how you show up fit and fresh.
No power meter? Use HR or pace
If you train without power, heart-rate TSS estimates load from time in your heart-rate zones, and pace-based TSS does the same for running off your threshold pace. They are approximations, but they keep the same 100-points-per-threshold-hour scale so your load history stays consistent. For a complementary view of your sustainable effort, the critical power calculator estimates the power you can hold before fatigue sets in.
Questions
What is a good weekly TSS?
It is personal and depends on your fitness, but as a rough guide many time-crunched amateurs sit around 300–500 TSS per week, committed age-groupers 500–800, and full-time athletes 1000+. What matters more than the absolute number is keeping the week-to-week change gradual — a sudden jump is what causes trouble.
What is the difference between TSS and Intensity Factor?
Intensity Factor (IF) is how hard a session was relative to your threshold — your normalized power divided by FTP. TSS combines that intensity with duration, so a short hard ride and a long easy ride can land on the same TSS by very different routes.
Can I get TSS without a power meter?
Yes. Heart-rate TSS (hrTSS) estimates load from time spent in heart-rate zones, and pace-based TSS (rTSS) does the same for running using threshold pace. They are less precise than power-based TSS but track training load well enough to manage fatigue.