Understanding TSS: Accurately assess your stress level with the Training Stress Score
After the ride, a number appears: 142 TSS. Is that a lot? Too much? Can you go full throttle again tomorrow – or do you need a rest day first? Anyone who trains without being able to answer these questions is flying blind. The Training Stress Score (TSS) is an attempt to condense training load into a single number – and it succeeds surprisingly well.
By Björn Kafka 4 minutes read time
Published: June 24, 2026
The concept originates from sports physician Andy Coggan, who developed it together with Hunter Allen for performance-based cycling training. Today, TSS is integrated into all common training software platforms – and those who understand it can use it to better assess individual training sessions as well as manage the overall training load over weeks and months.
What TSS actually measures
TSS combines two dimensions into one number: how intense a session was and how long it lasted. The principle follows a simple intuition: One hour at your FTP (Functional Threshold Power – the wattage you can sustain for approximately 60 minutes) corresponds exactly to 100 TSS . This hour serves as the reference point.
The calculation uses the so-called Intensity Factor (IF) – the ratio of your average power output to your FTP:
IF = Average Power ÷ FTP
A ride at 70% of FTP has an IF of 0.70. A threshold session at FTP power has an IF of 1.0. A short race with above-average effort can reach an IF above 1.0.
The TSS is then calculated from:
TSS = IF² × Duration (in hours) × 100
The quadratic component of the IF is crucial: intensity is not weighted linearly, but disproportionately. A session with an IF of 0.9 doesn't cost 10% more than one with an IF of 0.8, but rather a good 26% more (0.81 vs. 0.64). This reflects how the body reacts to high intensity: high-intensity training generates physiologically disproportionately more stress than moderate training.
TSS categories: What the number means
In practice, the following guidelines have become established: 1
| TSS | Load category | Example unit |
|---|---|---|
| < 50 | Relaxation/Light | Short basic training ride, active recovery |
| 50–100 | Moderate | 2 hours Zone 2, classic endurance ride |
| 100–150 | Demanding | 3-hour ride, medium intensity |
| 150–250 | Intensive | Race, long threshold unit |
| > 250 | Very hard | Granfondo, stage race day |
These categories are guidelines. An experienced rider with high basic endurance may digest a 150 TSS session differently than someone who is just building their base fitness.
Your TSS calculator
Enter your FTP, average power output of the ride and duration – the calculator will show you TSS and IF directly.
Determining FTP: The basis for precise values
TSS is only as accurate as your FTP value. Anyone using an outdated or incorrect FTP value will get skewed results.
The most common field method is the 20-minute maximum test : You give it your all for 20 minutes, multiply your average power output by 0.95 – and you get a good estimate of your FTP. More accurate values are provided by a graded exercise test in a lab or on an ergometer, ideally with lactate measurement.
Important: Your FTP changes with your training level. After an intensive training phase or a longer break, you should update it. Interval training and structured threshold sessions are typically the most effective methods for increasing FTP.
From TSS to CTL and ATL: The long-term perspective
A single TSS score says little about the overall situation. The real added value comes from summing and analyzing TSS scores over several weeks. Common training software (TrainingPeaks, Intervals.icu) calculates the following from this:
CTL (Chronic Training Load) – the average training load over the last ~42 days, weighted in descending order. Simply put: your fitness level. A high CTL means you've been training hard for an extended period.
ATL (Acute Training Load) – the workload of the last ~7 days. Simply put: your current level of fatigue. A single hard training block will significantly increase your ATL.
TSB (Training Stress Balance) – the difference between CTL and ATL. A positive TSB means you are fresher than your fitness level suggests; a negative TSB means accumulated fatigue. Race day targeting via TSB is a common method in structured performance training.
The classic principle: Build training blocks with increasing TSS (and decreasing TSB) – and then use targeted tapering phases to bring the TSB back into a positive range. Those who train solely by feel miss this opportunity.
TSS and Zone 2: Why low units still count
It may seem counterintuitive at first: A 3-hour ride at Zone 2 pace can easily reach 80–100 TSS – the same time, but with low IF. These sessions are essential for building basic endurance and metabolic fitness, and they contribute to CTL development without disproportionately burdening the fatigue balance.
The polarized training model – lots of Zone 2, few but very intense sessions – utilizes precisely this characteristic: a low TSS-per-hour rate during long base rides, a high TSS rate during short high-intensity sessions. Combined, this results in a high weekly TSS without excessive exhaustion.
Limits of TSS
TSS is based on average power output and therefore cannot provide information about how power was distributed within the ride. A flat, sustained power output at an IF of 0.75 over 2 hours and a ride with the same average power output but many short sprints and long recovery phases will result in identical TSS – even though the neuromuscular load is very different.
Additional metrics such as Normalized Power (NP) or Variability Index help to capture this dimension. Nevertheless, TSS provides the best entry point into structured load management for most ambitious amateurs.
TSS Calculator (Training Stress Score)
Sources & References
- Allen, H. & Coggan, A. (2010). "Training and Racing with a Power Meter. VeloPress . Standardreferenz für TSS, IF und CTL/ATL".
- Banister, E.W. (1991). "Modeling elite athletic performance.In: MacDougall, J.D.et al.(Hrsg.) Physiological Testing of the High-Performance Athlete.Human Kinetics". [Hintergrund zum Impulse-Response-Modell].
- Stöggl, T. & Sperlich, B. (2014). "Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. Frontiers in Physiology". DOI: 10.3389/fphys.2014.00033. https://doi.org/10.3389/fphys.2014.00033
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