Training Metrics (Formulas)

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.

TSS — Training Stress Score

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:

1. Power TSS — from normalized power and FTP (most accurate, needs a power meter)
2. Heart rate TSS — estimated from heart rate using the TRIMP formula
3. Duration TSS — estimated from ride time and elevation (always available)

Every ride gets a TSS value regardless of what sensors you have.

CTL — Chronic Training Load (Fitness)

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:

• Under 20 — just starting out
• 20–40 — recreational rider
• 40–60 — regular rider with a solid base
• 60–80 — well-trained
• 80+ — competitive fitness

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."

ATL — Acute Training Load (Fatigue)

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.

TSB — Training Stress Balance (Freshness)

What it is: Rested or tired. The balance between fitness and recent fatigue.

How it's calculated: TSB = CTL − ATL.

• Above 15 — race ready
• 5 to 15 — well rested
• -10 to 5 — balanced
• -10 to -20 — tired
• Below -20 — very tired

FTP — Functional Threshold Power

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 set: RideTool imports FTP from Strava or Wahoo if available. If not, it estimates one from your best power efforts. You can also set it manually.

TRIMP — Heart-rate-based TSS

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.

1. HRR — what percentage of your heart rate range you used. With resting 60, max 180, average 140 on the ride, that's (140 − 60) ÷ (180 − 60) = 67%.
2. Exponential weighting — 90% HRR costs much more than 50%, not just proportionally more.
3. Duration — longer rides accumulate more stress.

A 90-minute moderate ride might score around 100–140 TSS from HR — similar to what you'd get from power.

W/kg — Power-to-Weight

FTP divided by body weight in kilograms. Single best predictor of climbing ability.