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Riegel Race Time Predictor Calculator

Predict race times at a new distance with Riegel

How it is calculated

The predicted time follows a power law of distance:

T2 = T1 × (D2 / D1)1.06

D1 and T1 are the distance and time of a recent race, D2 is the distance you want a prediction for. Enter both distances in the same unit (km or miles, it cancels out) and T1 in minutes, because the result is labelled in minutes. The exponent 1.06 is slightly above 1, so a distance twice as long takes a little more than twice the time. All three values must be above zero. The result is a decimal number of minutes, not hours and minutes.

Worked example

A 10 km race run in 50 minutes; predict a half marathon (21.0975 km).

  1. Distance ratio: 21.0975 / 10 = 2.10975
  2. Raise to 1.06: 2.109751.06 ≈ 2.2064
  3. T2 = 50 × 2.2064 ≈ 110.32 min

That is about 1 h 50 min 19 s, a pace of roughly 5:14 per km.

Try this example →

Common mistakes

  • Entering the time as 50.30 for 50 minutes 30 seconds: the field takes decimal minutes, so that would be 50.3 minutes; 50 min 30 s is 50.5.
  • Reading the answer as hours and minutes: 110.32 min is 1 h 50 min, not 1 h 10 min.
  • Mixing units between the two distances, such as 10 km and 13.1 miles.
  • Predicting a marathon from a 5 km race without matching training: the formula assumes equal endurance at both distances, so predictions across very different distances tend to be too optimistic.
  • Using a time from an easy training run instead of an all-out race effort.

About this calculator

This calculator applies Riegel's endurance formula, T2 = T1 × (D2/D1)^1.06, published in American Scientist in 1981 by Pete Riegel, a research engineer and marathoner, from analyzing race results across distances. The exponent 1.06 (rather than 1.0) captures the fact that pace naturally slows over longer distances due to fatigue and fueling limits, so times don't scale linearly with distance.

Runners use it to set a realistic goal pace for an upcoming race based on a recent result at a different distance — predicting a marathon time from a half-marathon PR, for instance — though it works best between distances that aren't too far apart, since the formula assumes similarly trained endurance at both.

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