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Race Time Predictor

A time you have already run says a great deal about the times you could run at other distances — as long as the model's one assumption is visible. Riegel's formula scales time with distance raised to a fatigue exponent, and that exponent is the whole model: 1 would mean holding the same pace forever, and the usual 1.06 means pace slips slightly as the distance grows. This exposes it rather than burying it.

What this generator does

Applies Riegel's formula to scale your known time to each standard distance, and then runs each prediction back through the formula to recover the time it started from — which confirms the arithmetic even though it says nothing about whether the model fits you.

How to use this tool

  1. Enter a distance you have run and your time for it.
  2. Adjust the exponent if you know your endurance is unusual.
  3. Read the predicted times and paces.
  4. Note that every prediction converts back to your original time.

Understanding the controls

Distance and time
What you have actually run. A recent, hard effort predicts better than an old one or a training run.
Fatigue exponent
In hundredths, so 106 means 1.06. Lower it towards 1.03 for a well-trained endurance runner and raise it towards 1.10 if you have never raced beyond 10 km.

Common use cases

  • Estimating a marathon time from a recent half
  • Setting a target for a distance you have not raced
  • Comparing two results at different distances on equal terms
  • Seeing how much the fatigue exponent changes a long prediction
  • Checking a coach's target against the standard model

How this generator works

Riegel's formula multiplies your time by the ratio of the distances raised to the exponent, so a distance twice as long takes a little more than twice as long. Every prediction is then run back through the same formula in the opposite direction, and must return the time you entered. Longer races are also confirmed to be predicted slower per kilometre whenever the exponent is above one, which is the behaviour the model is meant to have.

Randomness and fairness

Nothing here is random. A time, a distance and an exponent give exactly one set of predictions. Nothing here is medical or coaching advice; these are arithmetic models, and how you train is a matter for you and anyone qualified you ask.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

  • The model assumes you are equally trained for every distance, which almost nobody is — a fast 5k predicts an optimistic marathon for most people.
  • Terrain, weather and course profile are all outside it, and any of them can cost more than the model's whole margin.
  • Predictions above the marathon become unreliable, since ultra performance depends on far more than aerobic fitness.
  • A single exponent cannot describe a runner whose endurance differs from their speed.Nothing here is medical or coaching advice; these are arithmetic models, and how you train is a matter for you and anyone qualified you ask.

Privacy and your data

The prediction is computed in your browser. Your distance and time are never transmitted, stored or included in analytics.

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