Training Stress Score (TSS) Calculator

FTP appears twice in the formula: once directly in the denominator, and again indirectly through the Intensity Factor (normalized power divided by FTP), which is then multiplied back in. Because of that double role, overestimating your FTP doesn't just shift the score a little — it systematically deflates every TSS you calculate afterward, since a higher FTP makes every ride look like a smaller fraction of your threshold effort.

Average power treats a steady effort and a spiky, stop-and-go ride as equivalent whenever the raw wattage average matches, but the physiological cost of surging and coasting is much higher than steady output. Normalized power is designed to weight those surges more heavily, so it better reflects the true metabolic demand — feeding a simple average into this formula for a punchy or interval-heavy ride will understate how hard the ride actually was.

Intensity Factor is normalized power divided by FTP, so a value of 1.0 means your normalized power for the ride equaled your functional threshold power exactly — the effort you could theoretically sustain for about an hour. An IF above 1.0 signals an unsustainably hard pace for that duration, while endurance rides typically sit well below 0.75.

Recovery Hours is a simple heuristic — TSS divided by 10, rounded, then doubled — and the Weekly TSS Target is just FTP multiplied by 3.5, both meant as rough planning benchmarks rather than physiological measurements. Real recovery time depends heavily on sleep, nutrition, training history, and how the effort was distributed, so treat these two outputs as starting points to adjust from, not prescriptions.