CTL stands for chronic training load, and it is the number most training platforms use to represent your fitness.
It is calculated as a rolling weighted average of your daily training stress score across 42 days, with recent days counting for slightly more than older ones.
The long window is the whole point. Because it averages six weeks, a single enormous weekend barely moves your CTL, and a single missed week barely dents it.
That makes it a measure of the training you have actually absorbed, rather than the training you did on Saturday.
It also explains something that worries riders more than it should: fitness is slower to lose than it feels. A fortnight away from the bike pulls CTL down visibly, but nothing like the collapse people imagine, and it comes back faster the second time.
CTL, ATL and TSB: the three numbers together
CTL is never read alone. It is one of three numbers that come from the same daily training stress score, and every training platform shows them on the same chart.
| Metric | Window | What it represents |
|---|---|---|
| CTL, chronic training load | 42 days | Fitness. Moves slowly |
| ATL, acute training load | 7 days | Fatigue. Spikes and falls fast |
| TSB, training stress balance | CTL minus ATL | Form. Positive is rested, negative is loaded |
A season that is going well looks like CTL rising gradually, ATL spiking and recovering around hard weeks, and TSB swinging negative during a build and positive before an event.
The limitation worth knowing
CTL inherits the weakness of the metric underneath it: training stress score is measured against your FTP.
If your FTP is three months old and you have improved since, every ride has been scored too high, so your CTL is overstated and your TSB is telling you that you are more tired than you are.
Retesting is what keeps the whole chart honest.
If you want the number to go up, how to increase CTL works through the week-by-week figures of a real eight-week plan, including the week where it is supposed to fall.