Heart-rate-based Training Stress Score quantifies a session's stress from how long you trained and how hard, relative to your lactate threshold heart rate. One hour at exactly threshold equals 100 TSS by definition.

How is each session scored?

The tool computes an intensity ratio of average heart rate to threshold heart rate, squares it, and multiplies by the duration in hours and by 100. Longer and harder runs therefore score higher.

What is lactate threshold heart rate?

LTHR is the heart rate at the intensity you could just sustain for roughly an hour all-out. A common field estimate is your average heart rate over the final 20 minutes of a hard 30-minute time trial.

Why does hrTSS underrate short intervals?

Heart rate is slow to respond, so during brief, very hard intervals your average HR never reaches the true effort level. For interval-heavy sessions a power or pace-based TSS reflects the load more accurately.

How fast should I increase weekly load?

A widely used guideline is to raise weekly load by no more than about five to ten percent week on week. Sharp jumps in total TSS are strongly associated with overuse injuries.

Running Weekly Training Load Calculator

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Put a number on how hard your week was

Two runners can both log “60 miles a week” and carry completely different training stress, because intensity matters as much as volume. Training Stress Score collapses both into one figure so you can compare a hard week to an easy one, spot when you are ramping too quickly, and plan recovery deliberately. This tool builds your weekly total from each run’s duration and average heart rate.

How it works

For each run the tool derives an intensity ratio from your average heart rate relative to your lactate threshold heart rate, then converts duration and intensity into a stress score:

hrRatio = avgHR / LTHR
hrTSS   = durationHours * hrRatio^2 * 100

Squaring the ratio means harder efforts count disproportionately more, which matches how fatigue accumulates. By definition, one hour spent exactly at threshold heart rate (hrRatio = 1) scores 100. The weekly total is simply the sum of every session’s hrTSS, and the tool classifies the result into recovery, maintenance, productive, or overreaching bands.

Tips and notes

Get your LTHR right first, because every score scales off it — a value that is too low inflates every session. The hrTSS method shines for steady aerobic and tempo running but understates short, sharp intervals, where heart rate never catches up to the real effort; reach for power or pace-based load on those days. Use the weekly total to manage progression: a gentle, consistent climb builds fitness, while repeated large jumps are one of the clearest warning signs of impending overuse injury.