# PSS — Power Stress Score

Canonical HTML: https://effortsignal.com/learn/pss/

Last content update: 2026-09-10

PSS combines timer duration with weighted power relative to the applicable FTP.

## Formula and inputs

```text
PI = Weighted Power / effective FTP
PSS = (timer seconds / 3600) × PI² × 100
```

Weighted Power and FTP are in watts; PSS is a score. One hour at PI = 1 yields 100 PSS. The activity's effective threshold profile supplies cycling FTP, an explicitly configured indoor FTP for an indoor ride, or running threshold power for running. A run does not inherit cycling FTP. A threshold whose effective date is later than the activity is not automatically valid for that earlier recording.

## Verified example

A one-hour constant-power recording at 200 W with effective FTP 250 W has Weighted Power 200 W, PI 0.8 and PSS 64. A 30-minute recording with the same output and threshold has PSS 32.

## Pauses, zeros and missing data

The [Weighted Power](/learn/weighted-power/) calculation uses complete 30-second windows on compressed timer time and needs at least 80% power coverage. Real zero watts count as coasting; absent power remains missing. The positive recorded timer/moving duration supplies the PSS duration, with projected timer duration as fallback. Unsupported sport, unavailable weighted power or no valid positive applicable FTP prevents PSS. A short recording can carry lower confidence.

## Meaning and recalculation

PSS is threshold-relative: changing the applicable FTP changes the score even for unchanged watts. It is not interchangeable with [TRIMP](/learn/trimp/). PSS appears in activity, volume, planning and progress contexts, but does not feed EffortSignal's [CTL/ATL history](/learn/training-load/). The [Readiness workload component](/learn/readiness/#workload) has its own documented use of PSS.

## Sources and implementation scope

[Coggan, power-based training metrics](https://www.trainingpeaks.com/learn/articles/normalized-power-intensity-factor-training-stress/) provides background, not a specification of every EffortSignal rule. The formulas, windows, quality gates and examples above describe EffortSignal 0.1.13, checked September 10, 2026. Product-specific thresholds and rounding should not be attributed to the background source.