May 26, 2026 · 2 min read

What is TSS — Training Stress Score explained

TSS (Training Stress Score) is a single number that tells you how hard a training session was — factoring in both duration and intensity relative to your FTP. It was designed by Dr Andrew Coggan as part of his power-based training framework and is the foundation for managing training load with tools like PMC.

How is TSS calculated?

The formula uses three intermediate metrics:

• Normalised Power (NP) — a weighted average of your power output that accounts for the physiological cost of surges and variability. It's always equal to or higher than simple average power.
• Intensity Factor (IF) — the ratio of NP to your FTP (IF = NP / FTP). An IF of 1.0 means you rode at threshold the entire session.
• Variability Index (VI) — the ratio of NP to average power (VI = NP / Avg Power). A perfectly steady ride has a VI of 1.0; more surges mean a higher VI.

The actual TSS formula: TSS = (duration in seconds × NP × IF) / (FTP × 3600) × 100. In simple terms: one hour at exactly FTP = 100 TSS.

What do the numbers mean?

Some reference points for a single session:

• Under 150 TSS — a normal training day. Recovery within 24–48 hours.
• 150–300 TSS — a hard or long ride. May need two days of recovery.
• 300–450 TSS — very demanding. Expect significant fatigue lasting several days.
• Over 450 TSS — extreme. Typical for multi-hour endurance events or stage races.

TSS from heart rate (hrTSS)

When you don't have a power meter, training load can be estimated from heart rate using hrTSS. It's less precise — heart rate doesn't capture short surges the way power does — but it's far better than nothing. WattLog.pro calculates hrTSS automatically when power data is unavailable but heart rate is present.

Why does TSS matter?

TSS is the input currency for the PMC chart. Every day's TSS feeds into Fitness (CTL), Fatigue (ATL), and Form (TSB). Without consistent TSS tracking, you can't plan progressive overload or taper effectively. Consistency matters more than absolute accuracy — track every session, and the trends become meaningful over weeks.

← All blog posts