One hour ridden exactly at FTP is 100 TSS by definition. Intensity factor is simply normalised power divided by FTP.
Let it add up on its own WattLog.PRO computes TSS for every ride and tracks the rolling load behind it, so you can see fitness and fatigue building instead of scoring sessions one at a time.
Open WattLog.PROWhat TSS actually measures
Training stress score combines how hard a ride was with how long it lasted, into one number describing the physiological cost. The anchor is simple: one hour at exactly your FTP scores 100. Everything else scales from there, so a gentle three-hour endurance ride and a savage 45-minute interval session can produce similar scores by very different routes.
The calculation uses normalised power rather than plain average power, because a ride with hard surges is more costly than a steady ride at the same average. Normalised power weights the high-intensity portions to reflect that. The intensity factor - normalised power divided by FTP - then describes how hard the ride was independently of its length.
Reading the numbers
As a rough guide, under 150 TSS leaves you fresh the next day, 150 to 300 usually costs some residual fatigue, and above 300 typically needs a genuine recovery day. Intensity factor is the companion figure: below 0.75 is endurance riding, 0.85 to 0.95 covers tempo and sweet spot, and anything above 1.0 is a hard effort that cannot be sustained for long.
TSS is only as good as your FTP. If your threshold is out of date, every score is systematically wrong - an inflated FTP makes hard rides look easy and hides accumulating fatigue. The value of TSS also comes from the trend rather than the single ride: a sustainable weekly total that you can repeat beats an impressive one you need a fortnight to recover from.
Frequently asked questions
What is a good weekly TSS?
It depends entirely on your training history. Recreational riders often accumulate 300 to 500 a week, committed amateurs 500 to 800, and full-time racers well over 1000. The right figure is one you can repeat week after week without your form going backwards.
Can I calculate TSS without a power meter?
Yes, by substitute. Heart-rate-based TSS uses time in heart rate zones, and there are speed-based estimates too. Both are cruder than the power-based figure but track training load well enough to be useful. WattLog.PRO falls back through power, then heart rate, then speed automatically.
What is the difference between normalised and average power?
Average power is the plain mean. Normalised power weights hard efforts more heavily to reflect their disproportionate physiological cost. On a steady ride the two are nearly identical; on a surging group ride or a criterium, normalised power can be far higher.