GPXFIT

TSS Calculator

The TSS Calculator calculates Training Stress Score (TSS) metrics from your activity data, following the TrainingPeaks methodology developed by Dr. Andrew Coggan.

What is TSS?

Training Stress Score (TSS) is a single number that represents the total training load of a workout, taking into account both duration and intensity. It helps you:

  • Quantify workout difficulty on a consistent scale
  • Plan recovery between workouts
  • Track cumulative training load over time
  • Avoid overtraining

Key reference: 1 hour at your threshold (FTP, LTHR, or Threshold Pace) = 100 TSS

TSS Calculation Methods

This tool calculates TSS using up to three different methods, depending on what data is available in your file:

Power TSS (pTSS) - Most Accurate

Available when your file has power meter data.

Key metrics:

  • Normalized Power (NP): A weighted average that better reflects the physiological cost of variable power output. Calculated using 30-second rolling averages raised to the 4th power.
  • Intensity Factor (IF): NP ÷ FTP. Represents workout intensity as a percentage of threshold.
  • Variability Index (VI): NP ÷ Average Power. Higher values indicate more variable pacing.

Formula: TSS = (Duration × NP × IF) ÷ (FTP × 3600) × 100

Heart Rate TSS (hrTSS)

Available when your file has heart rate data.

Two methods available:

Zone-based (simpler):

  • Assigns TSS per hour based on time in each HR zone
  • Only requires LTHR setting
  • Good for steady-state efforts

TRIMP (more accurate):

  • Uses exponential weighting based on heart rate reserve
  • Requires LTHR, Max HR, and Resting HR
  • Better for variable-intensity workouts

Pace TSS (rTSS) - Running

Available for running activities with pace/speed data.

Key metrics:

  • Normalized Graded Pace (NGP): Adjusts pace for terrain gradient. Running uphill at 5:00/km is harder than running flat at the same pace.
  • Grade Adjustment: Approximately +3.3% metabolic cost per 1% uphill gradient

Formula: Same as pTSS but using NGP instead of NP

Setting Your Threshold Values

FTP (Functional Threshold Power)

Your FTP is the maximum power you can sustain for approximately one hour.

How to find it:

  1. Do a 20-minute all-out effort
  2. Take your average power for those 20 minutes
  3. Multiply by 0.95

Typical range: 150-400 watts depending on fitness and body weight.

LTHR (Lactate Threshold Heart Rate)

Your LTHR is the heart rate you can sustain for about one hour during an all-out effort.

How to find it:

  1. Do a 30-minute solo time trial (best effort)
  2. Record your average heart rate for the last 20 minutes
  3. That's your LTHR

Typical range: 150-185 bpm depending on age and fitness.

Max HR (Maximum Heart Rate)

Your absolute maximum heart rate. Used for TRIMP calculations.

How to find it:

  • Use the highest HR you've ever recorded during exercise
  • Or estimate: 220 - age (rough approximation)

Resting HR (Resting Heart Rate)

Your heart rate at complete rest. Used for TRIMP calculations.

How to measure:

  • Measure first thing in the morning, before getting out of bed
  • Take an average over several days

Typical range: 40-70 bpm depending on fitness level.

Threshold Pace (Running)

Your best sustainable pace for approximately one hour.

How to find it:

  • Your 10K race pace is close to threshold pace
  • Or: 5K pace + 15-20 seconds per km/mile

Interpreting TSS Values

TSS Level Recovery Time Description
< 150 Low 1-2 days Routine training, recovery possible next day
150-300 Medium 2-3 days Noticeable fatigue, some residual tiredness
300-450 High 3-5 days Significant fatigue, requires extended recovery
> 450 Very High 5-7+ days Deep recovery needed, race-level effort

Tips for Using TSS

  1. Track weekly TSS: Most athletes can handle 400-700 weekly TSS depending on experience
  2. Build gradually: Increase weekly TSS by no more than 10% per week
  3. Include recovery: Plan easy days/weeks to absorb training stress
  4. Compare methods: If you have multiple data types, comparing TSS values can reveal insights about your pacing

Limitations

  • hrTSS can lag: Heart rate responds slowly to intensity changes, underestimating interval workouts
  • pTSS is most accurate: Power responds instantly to effort changes
  • rTSS needs elevation: Grade-adjusted pace requires accurate elevation data

References