Running Time Predictor

Use a recent race or time trial to predict what you could run at other distances, from 1 mile to the marathon, with Riegel's endurance formula.

Your details

h:mm:ss

Use a recent all-out race or time trial. Type the time as h:mm:ss or mm:ss.

Your results

Enter a recent race result to predict your time at other distances.

How to use it

  1. Choose km or miles, then pick the distance of a recent race, or type your own.
  2. Enter your finish time. Use an all-out effort from the last few months.
  3. Pick the distance you want to predict. The table shows predictions for every standard distance.

How it's calculated

The formula. Peter Riegel (1981) found that world-record times across running, swimming, cycling and walking follow the same pattern:

T₂ = T₁ × (D₂ ÷ D₁)^1.06

T₁ is your time over distance D₁, and T₂ is the predicted time over distance D₂. The calculator uses 1.06 as a practical approximation. Riegel reported about 1.08 for world-class runners and about 1.05–1.06 for older male groups. With either exponent, pace slows as the distance goes up.

Example: a 20:00 5K predicts 20:00 × (10 ÷ 5)^1.06 = 41:42 for 10K.

Range. Riegel fitted the formula to performances lasting about 3.5 to 230 minutes. The calculator flags predictions when the input race or predicted time falls outside that range. Above 230 minutes, time-distance curves bend upward, so long predictions tend to be too fast.

Limits. The prediction assumes you’re as well trained for the new distance as for the race you entered. A strong 5K runner who hasn’t done long runs won’t hit the predicted marathon time. Heat, hills and pacing also matter on the day.

Sources

  1. Riegel, P. S. (1981). Athletic records and human endurance. American Scientist, 69(3), 285–290.
  2. World Athletics. Half marathon: a road race of 21.0975 km, exactly half the marathon distance of 42.195 km.

Questions

How accurate is the prediction?

It assumes you're equally trained for both distances. Predictions to a nearby distance (5K to 10K) are usually close. Predictions from a short race to the marathon are often too fast, because the marathon needs endurance that a 5K doesn't test.

Which race should I use?

A recent all-out race or time trial, ideally the one closest to the distance you want to predict. A half marathon predicts a marathon better than a 5K does.

Why does pace slow down over longer distances?

You can't sustain the same intensity for longer. Riegel found that across world records, time grows slightly faster than distance, which a power-law exponent greater than 1 captures. The calculator uses 1.06 as a practical approximation.

Why are some rows greyed out?

Riegel fitted his formula to performances lasting 3.5 to 230 minutes. The calculator greys out predictions if the input race or predicted time falls outside that range.

Updated November 9, 2026.