Last updated: 2026-06-21

Putting a Number on a Run

Two runs can be hard in completely different ways. A 25-minute interval session and a two-hour long run both leave you tired, but they stress the body differently and they are awkward to compare with stopwatch time alone. Training Stress Score (TSS) solves that by folding duration and intensity into one figure, so a week of varied workouts adds up to a single, honest measure of how much training you have done.

This calculator estimates TSS for a single run from two things you already know: how long it lasted and how its pace compares to your threshold pace.

How It Works

The first step is the intensity factor, the ratio of your threshold pace to the pace you actually ran. Because pace is "time per distance," a faster run is a smaller number, so dividing threshold pace by run pace gives a value above 1 when you ran harder than threshold and below 1 when you ran easier:

The score then multiplies the duration in hours by the square of that intensity factor, scaled so that an hour at threshold lands on 100:

Squaring the intensity is what makes hard running count for so much more than easy running. Doubling the time doubles the score, but running a little faster raises it sharply.

Worked Example

Suppose your threshold pace is 4:30 per kilometre and you run for one hour at 5:00 per kilometre — comfortably below threshold.

The intensity factor is $4.5 \div 5.0 = 0.9$. Over one hour the score is $1 \times 0.9^2 \times 100 = 81$ TSS. Pick up the pace to threshold, 4:30 per kilometre, and the same hour scores the full 100. With a threshold of 7:15 per mile and a run at 8:03 per mile, the intensity factor is about $7.25 \div 8.05 = 0.9$. One hour at that effort scores $1 \times 0.9^2 \times 100 = 81$ TSS, while a full hour at threshold pace scores 100.

Reading the Result

• Around 50 or below is a short or very easy session — a recovery jog or a brief shakeout.
• Roughly 60 to 120 covers most quality workouts and moderate long runs.
• Above 150 usually means a long run or a hard, sustained effort that will need real recovery.

Because the score compounds across the week, the most useful habit is watching the weekly total trend upward gradually and easing off before it climbs too fast.

Where Pace-Based TSS Falls Short

The score assumes pace is a fair proxy for effort, which holds best on flat, steady runs. On a hilly route, into a headwind, or in heat, your pace drops even when you are working hard, so the calculator can understate the true load. On those days, trust how the run felt as much as the number — and if you train with a power meter or heart rate, treat this as one estimate among several.