What is TSS, really?

Training Stress Score is the most useful number a cyclist or runner has, and the most misunderstood. It's a single integer that says how hard a workout was, in a way that's comparable across days, sports, and athletes.

It's also, at the time of writing, behind a paywall on Strava and a $240/yr subscription on TrainingPeaks.

It shouldn't be. The formula is one line. This post walks through it from first principles, computes it on a real FIT file, and shows you how to read it.

The problem TSS solves

Two workouts:

A: 60 minutes at threshold.
B: 90 minutes at endurance.

Which was harder? "Harder" how — perceived effort? Calorie burn? Recovery cost?

The honest answer is recovery cost. You can do workout B almost every day. You can't do workout A two days in a row without paying for it. We need a number that reflects that asymmetry.

The trick is that recovery cost doesn't scale linearly with intensity. Doubling power doesn't double soreness — it more than doubles it. So a useful score has to weight intensity non-linearly.

That's exactly what TSS does.

The formula

TSS = (seconds × NP × IF) / (FTP × 3600) × 100

Where:

seconds — duration of the workout in seconds.
NP — normalized power, a "perceived" average that punishes variability.
IF — intensity factor, equal to NP / FTP.
FTP — your functional threshold power, the wattage you can hold for ~1 hour.

A 1-hour ride at exactly your FTP scores TSS = 100 by definition. That's the anchor.

Examples:

60 min at 0.85 IF (sweet spot) → TSS ≈ 72
90 min at 0.70 IF (endurance) → TSS ≈ 73 (yes, almost identical)
30 min at 1.10 IF (threshold over-unders) → TSS ≈ 60
5 hour at 0.65 IF (long endurance) → TSS ≈ 211

That last one is why long rides crush you for two days, and the threshold over-unders only knock you out for one.

Where Normalized Power comes from

The naïve thing is to take the average wattage. That fails:

A steady 200W ride and a 0/400/0/400 sawtooth ride have the same average. The sawtooth is much harder.
Average power throws away the part that matters — the bursts.

Andrew Coggan's fix is to weight the intensity by itself, then take the mean. The standard implementation:

Smooth the power stream with a 30-second rolling average.
Raise each smoothed sample to the 4th power.
Take the mean of those.
Take the 4th root of the mean.

That's NP. The 4th-power weight makes high samples count disproportionately, which matches how the body experiences variable efforts.

In TypeScript:

function normalizedPower(watts: number[]): number {
  const window = 30;
  const smoothed: number[] = [];
  let sum = 0;
  for (let i = 0; i < watts.length; i++) {
    sum += watts[i];
    if (i >= window) sum -= watts[i - window];
    if (i >= window - 1) smoothed.push(sum / window);
  }
  const fourthPowerMean =
    smoothed.reduce((acc, w) => acc + w ** 4, 0) / smoothed.length;
  return fourthPowerMean ** 0.25;
}

That's the whole NP function. Source in pacelore.

Where Intensity Factor comes from

IF = NP / FTP. It's the share of your maximum sustainable effort the workout represented.

IF < 0.65 — recovery / Z2 endurance
IF 0.75–0.85 — tempo / sweet spot
IF 0.85–0.95 — threshold
IF 0.95–1.05 — VO2-max intervals
IF > 1.05 — race or all-out efforts

If you find yourself averaging IF > 0.85 over a four-hour ride, your FTP is probably set too low.

Why does Strava paywall this?

Two reasons, both economic:

Power-data users skew toward serious cyclists, who are more willing to pay for tools. So putting analytics behind a paywall converts well.
TrainingPeaks built its business on it. TSS, NP, IF, and the PMC chart are TrainingPeaks innovations (popularized by Andrew Coggan and Hunter Allen). Strava buying their way into that market — instead of building free competing implementations — is a strategic choice, not a technical one.

The cost to compute TSS is fractions of a millisecond per ride. The cost to show it to you is, generously, a tenth of a cent. There's no infrastructure reason it costs $240/yr. It costs $240/yr because the people who want it will pay it.

pacelore exists, in part, because that pricing isn't a law of nature.

How to read your TSS

Once you have it, the obvious next questions:

Daily TSS — how hard was today.
Weekly TSS — your training volume in load terms.
Chronic Training Load (CTL) — 42-day exponentially weighted average. Your fitness.
Acute Training Load (ATL) — 7-day EWMA. Your recent fatigue.
Training Stress Balance (TSB) — CTL − ATL. Negative means you're tired. Positive means you're rested (or detraining).

Together they form the Performance Manager Chart, which we cover in the next post.

Try it

pacelore computes TSS on every ingested ride and shows the breakdown — NP, IF, kJ, peak curves — without paywalls.

Live demo shows a real activity. Source is on GitHub.