TSB doesn't match CTL - ATL. Is this expected or a bug?

Kraisun Tunta

Hi everyone,

I’m trying to understand how Suunto calculates TSB (Training Stress Balance).

According to the common definition (e.g. TrainingPeaks),

TSB = CTL − ATL

However, my Suunto values don’t seem to match.

Here are some recent examples:
CTL ATL Suunto TSB CTL-ATL
29 46 -24 -17
28 39 -17 -11
29 48 -17 -19
29 41 -19 -12
30 44 -21 -14

In most cases, the displayed TSB differs by about 6-7 points from the simple CTL-ATL calculation.

I’ve already read several forum discussions mentioning that TSB may use yesterday’s CTL/ATL or internal decimal values, but even considering that, I’m still having trouble understanding these numbers.

Could someone clarify:
-Does Suunto calculate TSB differently from TrainingPeaks?
-Is TSB based on yesterday’s Fitness/Fatigue?
-Are the displayed CTL and ATL rounded while TSB uses hidden decimal values?
-Or is this a known issue in the current firmware/app?

I’m not complaining—I’m genuinely trying to understand the algorithm so I can interpret my training correctly.

Thanks!

Squirrel

@Kraisun-Tunta said:

According to the common definition (e.g. TrainingPeaks),

TSB = CTL − ATL

Actual Formula:

Form (TSB) = Yesterday’s Fitness (CTL) - Yesterday’s Fatigue (ATL)

https://help.trainingpeaks.com/hc/en-us/articles/204071764-Form-TSB

Kraisun Tunta

@Squirrel Thanks a lot for the clarification!

That actually makes much more sense now. I’ll record my CTL, ATL, and TSB values for the next 2–3 days and compare them to see if the numbers match yesterday’s CTL − ATL. Thanks again!