You don't need to know any of this to use RideTool, the Me card and emails tell you everything that matters in plain English. This page is the technical reference for the acronyms underneath: CTL, ATL, TSB, TSS, FTP.
What it is: A single number for how hard a ride was. A 1-hour easy spin might be 40 TSS. A 3-hour hard group ride might be 250 TSS.
How it's calculated: RideTool picks the best method available from your data:
Every ride gets a TSS value regardless of what sensors you have.
What it is: Your long-term fitness level. Think of it as "how much base have I built?" It goes up slowly when you train consistently and drops slowly when you stop.
How it's calculated: A 42-day rolling average of your daily TSS.
What the numbers mean:
Where it surfaces: CTL is the big number in the Fitness tile on the Me card and drives the smooth line in the Effort chart. Trending up over 28 days = "getting stronger." Dropping = "losing fitness."
What it is: How hard you've been training in the last 7 days.
How it's calculated: A 7-day rolling average of your daily TSS.
On its own, ATL doesn't tell you much. The signal is the balance between ATL and CTL, that's TSB.
What it is: Rested or tired. The balance between fitness and recent fatigue.
How it's calculated: TSB = CTL − ATL.
What it is: The maximum power (watts) you can sustain for about an hour. It's the baseline for calculating how hard any ride was relative to YOUR ability.
How it's managed: RideTool re-estimates your FTP continuously as you ride, from your best sustained power efforts, using several methods cross-checked against each other. When the evidence genuinely moves, the number moves; when it doesn't, the number holds. A value imported from Strava or Wahoo, or one you set yourself, is treated as input the estimate respects, not something it overwrites blindly.
It's date-aware. Every ride is scored against the FTP you had on that day, not today's number, so your history stays honest when your threshold changes. During an active injury the estimate is held rather than punished.
Why it can hold still while fitness climbs: fitness grows from all your riding, but FTP only moves on power proof, a sustained hard effort of roughly 20 minutes or more. Surge-and-coast trail riding rarely produces one. If your number feels stale, one steady 20 minute effort refreshes it honestly.
TRIMP (TRaining IMPulse) is the formula RideTool uses when there's no power data. It works from heart rate reserve and duration, weighted exponentially so harder efforts cost more.
A 90-minute moderate ride might score around 100–140 TSS from HR, similar to what you'd get from power.
FTP divided by body weight in kilograms. Single best predictor of climbing ability.