Training Stress Score Formula
Learn the Training Stress Score (TSS) formula used to quantify workout load.
Understand how TSS drives fitness and fatigue models.
Need to calculate, not just reference? Use the interactive version.
Open Cycling Training Stress Score Calculator →
The TSS Formula
TSS = (t × NP × IF) / (FTP × 3600) × 100
Where t is duration in seconds, NP is Normalized Power, IF is Intensity Factor, and FTP is your Functional Threshold Power.
Intensity Factor
IF = NP / FTP
Intensity Factor expresses how hard you worked relative to your threshold. An IF of 1.0 means you rode at exactly threshold intensity.
Variables
| Symbol | Meaning | Unit |
|---|---|---|
| TSS | Training Stress Score | dimensionless |
| t | Duration of the workout | seconds |
| NP | Normalized Power | watts |
| IF | Intensity Factor (NP / FTP) | dimensionless |
| FTP | Functional Threshold Power | watts |
Example 1 — Benchmark Ride
1-hour ride at exactly FTP (NP = FTP = 270 W).
IF = 270 / 270 = 1.0
TSS = (3600 × 270 × 1.0) / (270 × 3600) × 100
TSS = 100 — the definition of TSS
Example 2 — Long Endurance Ride
3-hour ride at NP = 195 W, FTP = 270 W.
IF = 195 / 270 = 0.722
TSS = (10800 × 195 × 0.722) / (270 × 3600) × 100
TSS ≈ 157 — solid aerobic training load
TSS Guidelines
| TSS per Workout | Recovery Needed |
|---|---|
| <150 | Ready next day |
| 150–300 | Mild fatigue for 1–2 days |
| 300–450 | Significant fatigue for 2–3 days |
| >450 | Deep fatigue, 3–5 days needed |
When to Use It
- Comparing the load of very different workouts
- Planning weekly training volume without overreaching
- Tracking fitness and fatigue with a Performance Management Chart
- Timing peak form for target races
Key Notes
- Formula: TSS = (duration_s × NP × IF) / (FTP × 3600) × 100: Where NP is Normalized Power, IF = NP/FTP (Intensity Factor), and FTP is Functional Threshold Power. A simpler approximation: TSS ≈ (duration_h × IF²) × 100.
- What TSS measures: TSS quantifies the physiological stress of a single workout on a 0–300+ scale. ~50 TSS = moderate effort; ~100 TSS = very hard day; completing a century ride might generate 250–300 TSS. Recovery time needed scales roughly with TSS.
- Chronic Training Load (CTL) and Acute Training Load (ATL): CTL (fitness) is the 42-day exponential average of daily TSS. ATL (fatigue) is the 7-day average. Form = CTL − ATL. Positive Form = fresh; negative Form = fatigued but potentially fit. Peaking for an event means raising CTL then reducing ATL.
- FTP is the anchor: All TSS calculations are relative to FTP (the power sustainable for ~60 minutes). If FTP is not accurately determined, TSS values are meaningless. Re-test FTP every 6–8 weeks as fitness changes.
- Applications: TSS-based training management is used in competitive cycling, triathlon, and running to plan training load, avoid overtraining, schedule rest days, and time peak performance for target events using periodization software like TrainingPeaks.