TSS (Training Stress Score) Explained — Manage Your Training Load with One Number

TSS (Training Stress Score) is a metric that condenses the load of a training session — its duration and intensity — into a single number. One hour of training exactly at your individual performance threshold equals 100 TSS by definition; anything below is easier, anything above is harder. With this one value, sessions, weeks and entire training blocks become comparable.

How is TSS calculated?

The simplified formula is: TSS = (duration in hours × intensity factor²) × 100. The intensity factor (IF) describes how hard the session was relative to your threshold — measured via power (watts), pace or heart rate. Three examples make this tangible:

• Easy run, 60 minutes at IF 0.70: about 50 TSS.
• Threshold workout, 45 minutes at IF 0.95: about 68 TSS.
• Long run, 2.5 hours at IF 0.75: about 140 TSS.

Two things stand out: first, intensity enters the formula squared — a slightly harder session produces considerably more stress. Second, duration drives the value up linearly. This is exactly why the long, easy runs in marathon training are the biggest TSS contributors even though they feel easy. And it is why running easy sessions "just a bit" too fast is so risky: the load price rises disproportionately.

CTL, ATL and TSB: fitness, fatigue, form

A single TSS value only becomes truly useful when viewed over time. Three derived quantities form the classic performance management model:

• CTL (Chronic Training Load) — the rolling average of your daily TSS over roughly 42 days. It describes your accumulated fitness: slow to rise, slow to fall.
• ATL (Acute Training Load) — the average over roughly 7 days. It reflects your current fatigue and reacts quickly to hard weeks or rest days.
• TSB (Training Stress Balance) — the difference between CTL and ATL, colloquially your "form". Slightly negative during build training (you are challenging yourself), slightly positive after a taper (you are fresh).

The logic behind it: fitness does not come from isolated heroic efforts but from the sum of many controlled stimuli. Raise your CTL calmly and steadily over months, and you will reach your goal faster than someone who trains hard in short waves and then has to stop with illness or injury.

TSS in practice: weekly load and ramp rate

In concrete terms, you use TSS on two levels. The first is weekly load: add up the TSS of all sessions in a week. Beginners often sit at 200–350 TSS per week, well-trained marathon runners at 400–700. More important than the absolute value is the progression — the so-called ramp rate. A robust rule of thumb: CTL should grow by no more than about 5–8 points per week, and weekly load by no more than roughly 10%. Larger jumps markedly increase the risk of injury and overtraining.

The second level is the distribution within the week. TSS makes visible whether you are really training according to the 80/20 principle: if your easy sessions are genuinely easy, a large share of your weekly TSS comes from volume, not intensity. If you run everything at medium pace instead, individual sessions inflate the value without fitness growing proportionally — lots of stress, little benefit.

The limits of TSS

As practical as the number is — it is a model, not a measuring device for your body. Three limits are worth knowing:

• Same TSS, different effect. 100 TSS of interval training stresses muscles and the nervous system differently than 100 TSS of easy endurance running. The number says nothing about the type of strain.
• No non-sporting load. Lack of sleep, job stress, heat or an oncoming cold do not show up in TSS, yet they massively change your actual capacity.
• Dependent on accurate thresholds. If your assumed threshold is outdated, all TSS values are systematically wrong. Threshold tests or recent race data are therefore part of the mandatory programme.

TSS does not replace body awareness — it complements it with an objective, comparable quantity.

How Peakora uses TSS in the recovery monitor

In the Peakora engine, TSS is the central measure of load: every planned and every completed session receives a TSS value, from which CTL, ATL and TSB are calculated automatically. The 24-hour recovery monitor cross-checks this training data with your recovery values — and adapts the plan when the load runs ahead of your current condition. This keeps the ramp rate in check without you having to do the maths, and weekly progression stays capped at 8–10%. Combined with the automatic 80/20 distribution, the result is exactly the load management that would be tedious and error-prone to do manually.

Frequently asked questions about TSS

What is a good TSS per week?

It depends on your training history: beginners often sit at 200–350 TSS per week, well-trained marathon runners at 400–700, professionals well above that. The absolute value matters less than the progression — weekly load should grow by only about 5–10% per week.

How do you calculate TSS?

In simplified terms: TSS = (duration in hours × intensity factor²) × 100. The intensity factor is the ratio of your effort to your individual threshold. One hour exactly at threshold yields 100 TSS; an hour of easy base running yields roughly 40–60 depending on pace.

What is the difference between TSS and CTL?

TSS measures the load of a single training session. CTL is the rolling average of your daily TSS over roughly 42 days and describes your accumulated fitness. TSS is the building block; CTL is the building.