Training Stress Score Formula
Learn the Training Stress Score (TSS) formula used to quantify workout load.
Understand how TSS drives fitness and fatigue models.
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