Training Stress Score (TSS): What It Is and How to Use It
Training Stress Score puts a single number on how hard a session was, combining duration and intensity. Here is how TSS is calculated for cycling, running, and swimming, what the numbers mean, and how it feeds the fitness and fatigue model that drives smart training.
What Training Stress Score measures
Training Stress Score, or TSS, answers a simple question with a single number: how much did that session take out of me? It does this by combining two things every workout has, how long it lasted and how hard it was, into one value you can track, compare, and add up over time.
The anchor is your threshold. By definition, one hour spent exactly at your threshold equals 100 TSS. From there everything scales. A long, easy endurance ride can rack up 120-140 TSS through sheer duration despite never feeling hard. A short, brutal interval session can hit 90 TSS in 45 minutes because the intensity is so high. That is the whole point of the metric: it lets you compare a two-hour Zone 2 ride and a 45-minute threshold session on the same scale, because it captures the combined cost rather than just time or just intensity.
How TSS is calculated
The formula relates session intensity to your threshold, then scales by how long you held it. For cycling, intensity comes from normalized power compared to your FTP. The ratio of the two is your intensity factor, and TSS rises with both the intensity factor and the duration. An hour at threshold gives an intensity factor of 1.0 and 100 TSS. An hour at 85% of threshold gives a lower intensity factor and proportionally less TSS.
The same logic applies across sports, only the intensity input changes:
- Cycling uses normalized power against FTP. This is the original and most precise version, because power is a direct measure of work.
- Running uses a threshold pace, often labelled rTSS. It compares your pace to your threshold pace, adjusting for grade where the platform supports it.
- Swimming uses a threshold swim pace, labelled sTSS, comparing your swim pace to your critical swim speed.
The common thread is that every version needs an accurate threshold for that sport. This is the load-bearing detail. If your FTP, threshold pace, or swim threshold is wrong, every TSS number built on it is wrong in the same direction. Get the threshold right first, with a test or a confident estimate from recent best efforts, and the rest of the model becomes trustworthy. The TSS calculator handles the math for all three sports, and the FTP calculator turns a test result into the FTP that cycling TSS depends on.
From single sessions to fitness and fatigue
TSS becomes powerful when you stop looking at one workout and start looking at the trend. Daily TSS rolls up into two rolling averages that together form the Performance Management Chart:
- CTL (Chronic Training Load) is a 42-day rolling average of daily TSS. It rises slowly and represents your accumulated fitness. Building CTL is the long game of training.
- ATL (Acute Training Load) is a 7-day rolling average. It responds quickly and represents fatigue. A few hard days spike it fast.
- TSB (Training Stress Balance), or form, is CTL minus ATL. Positive form means you are fresh, negative means you are carrying fatigue. You build fitness in a fatigued state and race in a fresh one.
This fitness-fatigue model is why TSS matters. It lets you ramp load gradually, see when fatigue is outrunning fitness, and time a taper so form peaks on race day. It is also the backbone of knowing when to deload, because a CTL that keeps climbing while form stays deeply negative is a clear sign you are accumulating more stress than you are absorbing.
How to use TSS without misusing it
TSS is a planning and trend tool, not a scoreboard. The most common way athletes get it wrong is treating a bigger number as automatically better and chasing TSS for its own sake. That is exactly how overreaching happens. The value is in three things, and none of them is a single big number:
The ramp rate. CTL should climb gradually. A common guideline is to avoid raising CTL by more than about 5-7 points per week over a sustained period. Faster than that and injury and burnout risk climb steeply.
The balance. Watch the relationship between fitness and fatigue, not just the volume. Deeply negative form for weeks on end is a warning, not a badge.
Consistency over heroics. Steady, absorbable weekly TSS beats a giant week followed by a forced rest week from exhaustion. The body adapts to what it can recover from.
One honest caveat: TSS does not capture everything. It does not know about life stress, poor sleep, or illness, all of which change how much a given session actually costs you. This is why TSS works best alongside recovery signals like HRV trends and resting heart rate. The number tells you the planned stress. Your recovery data tells you whether your body agrees. Read them together and TSS goes from an interesting metric to a genuinely useful guide for how to train.
