How to predict your race time
- Enter the distance of a recent race or all-out time trial (tap 5K, 10K, half or marathon).
- Enter your finish time as mm:ss or h:mm:ss.
- The predictions table shows your expected time and pace for every standard distance.
About the Riegel formula
In 1977 the engineer and marathoner Peter Riegel noticed that runners' times across distances follow a simple power law: T2 = T1 × (D2 ÷ D1)1.06. The exponent above 1 captures how pace slows as distance increases. For example, a 25:00 5K predicts about 52:07 for 10K, 1:55:00 for the half marathon and just under 4:00:00 for the marathon.
The predictions assume you are equally well trained for the target distance. They are reliable from about 1,500 m up to the half marathon. For the marathon they are often optimistic, because fuelling, long-run training and pacing matter much more; many coaches add 5–10% for a first marathon. Use a recent race (within six to eight weeks) run on a similar course and in similar weather. Everything is calculated in your browser.
ℹ️ Not medical advice: an estimate for healthy adults. Ask a doctor or qualified professional about your own situation.
FAQ
Which race should I base the prediction on?
The longest recent race you ran at full effort; a 10K or half predicts a marathon better than a 5K.
Why is my real marathon slower than predicted?
Riegel assumes marathon-specific endurance. Low weekly mileage often adds 10% or more.
Does it work for trail races?
Only roughly. Hills and terrain change pace too much for a distance-only formula.