Training Stress Score Calculator

By Thomas Mortelmans

📋 What is TSS?

TSS puts a single number on how hard a workout was.

TSS 100 = one hour at your threshold

TSS	Example Session
20–40	Easy 1-hour recovery ride
50–80	1-hour endurance ride
100	1 hour at FTP (by definition)
120–150	Hard 90-min interval session
200–300	3–4 hour hard group ride
300–450	Century ride or hard stage race
450+	Ironman bike leg, epic day

🎯 STEP-BY-STEP GUIDE

Power-Based TSS (Most Accurate)

Enter:

Duration (minutes)
Normalized Power (from your cycling computer)
FTP (Functional Threshold Power)

Heart Rate TSS

Enter:

Duration (minutes)
Average HR (bpm)
LTHR (Lactate Threshold HR)
Resting HR and Max HR (optional, improves accuracy)

Pace-Based TSS (Running/Swimming)

Enter:

Duration (minutes)
Average Pace (min/km or min/100m)
Threshold Pace

If you want to see what I read to keep my own work sharp outside this newsletter, this is the list.

🔬 THE TSS FORMULA

TSS = (Duration_sec × NP × IF) / (FTP × 3600) × 100

Where IF = NP / FTP

Worked Example

90 min ride, NP = 230W, FTP = 250W

IF = 230 / 250 = 0.92
TSS = (5400 × 230 × 0.92) / (250 × 3600) × 100
    = 1,143,360 / 900,000 × 100
    = 127

📊 RECOVERY INTERPRETATION

TSS	Stress	Recovery Outlook
< 150	Low	Most trained athletes resume quality next day
150–300	Moderate	24–48 h before high-intensity, depends on CTL and IF
300–450	High	36–72 h. Glycogen status is the limiting variable
> 450	Very high	Event-level. 48–96 h, governed by fuelling and sleep

⚠️ Important Caveats

Two sessions with the same TSS can produce very different fatigue:

5 hours at IF 0.78 → ~300 TSS (mostly aerobic, moderate glycogen cost)
2.5 hours at IF 0.98 → ~300 TSS (high neuromuscular strain, massive glycogen depletion)

Recovery also depends on:

CTL (Chronic Training Load): CTL 110 athlete absorbs 300 TSS in 24–36 h. CTL 50 athlete needs 48–72 h.
Carb intake: Glycogen restoration = ~24 h at 6–10 g/kg/day. Insufficient fuelling = 48+ h.
Intensity distribution: Supra-threshold work creates more central fatigue
Sleep, heat, age, cumulative fatigue: All modify recovery

Interpreting Your Score:

TSS Breakdown:

<50:   Recovery/Easy
50-100: Endurance
100-150: Tempo/Sweet spot
150-200: Threshold work
200-300: Hard intervals
300+: Race or very hard session

Weekly TSS Targets:

Beginner: 200-400 per week
Intermediate: 400-700 per week
Advanced: 700-1000 per week
Elite: 1000+ per week

🔬 Scientific Basis

Key Research:

Coggan (2003) - "Training and Racing with a Power Meter"
TrainingPeaks (2006) - "Performance Management Chart"
Bannister (1991) - "Modeling elite athletic performance"

The Power-Based Formula:

TSS = (duration_seconds × NP × IF) / (FTP × 3600) × 100

Where:
IF (Intensity Factor) = NP / FTP

Why This Works:

Duration matters: 2 hours at FTP = 200 TSS (by definition)
Intensity matters: Harder efforts = exponentially more stress
IF squared: Captures non-linear fatigue

Example Math:

90 min at NP 230W, FTP 250W:

IF = 230 / 250 = 0.92
TSS = (5400 sec × 230W × 0.92) / (250W × 3600) × 100
    = 1,143,720 / 900,000 × 100
    = 94 TSS

Heart Rate-Based Formula:

hrTSS = duration_minutes × HR_ratio^1.92 × 100 / 60

Where:
HR_ratio = (Avg_HR - Rest_HR) / (Max_HR - Rest_HR)

Why the 1.92 exponent?

Empirically derived to match power-based TSS
Accounts for cardiac drift and non-linearity

Pace-Based Formula:

rTSS = duration_minutes × pace_ratio^2 × 0.1

Where:
pace_ratio = threshold_pace / avg_pace

Applications:

Performance Management Chart (PMC):
- CTL (Chronic Training Load): 42-day rolling average (fitness)
- ATL (Acute Training Load): 7-day rolling average (fatigue)
- TSB (Training Stress Balance): CTL - ATL (form)
Periodization:
- Build phase: Increase weekly TSS gradually
- Peak phase: Maintain TSS with higher intensity
- Taper: Reduce TSS 40-60%

Limitations:

Doesn't account for terrain (climbing)
Doesn't capture heat stress
Individual recovery varies
Best used as relative metric

Disclaimer

The information provided in this newsletter is for educational purposes only and does not constitute medical advice. Exercise physiology is highly individual; what works for elite populations may not apply to everyone. Always consult with a physician before making significant changes to your training, nutrition, or supplementation protocols. The Scientist's Notebook and ESQ Coaching accept no liability for injuries or health issues arising from the application of these concepts.