How the Riegel formula works
The Riegel formula, developed by Peter Riegel and published in 1977, is the gold standard for predicting race finish times across distances. The formula is: T2 = T1 × (D2 ÷ D1) ^ 1.06. The exponent 1.06 represents fatigue — as distance increases, your pace slows at a predictable rate relative to your shorter-distance performance.
If you ran a 45:00 10K, the formula predicts a 1:39:25 half marathon and a 3:27:18 marathon. These are physiological predictions, not goals — they assume you are equally well-trained for both distances.
Race prediction chart — from 10K time
| 10K time | Half Marathon | Marathon |
|---|---|---|
| 35:00 | 1:17:26 | 2:41:05 |
| 40:00 | 1:28:29 | 3:04:07 |
| 45:00 | 1:39:25 | 3:27:18 |
| 50:00 | 1:50:32 | 3:50:38 |
| 55:00 | 2:01:35 | 4:14:07 |
| 1:00:00 | 2:12:38 | 4:37:43 |
Frequently asked questions
What is the Riegel formula?
The Riegel formula predicts race time: T2 = T1 × (D2/D1)^1.06. The exponent accounts for fatigue over longer efforts. It was derived from analysis of world record progressions across distances.
How accurate is the Riegel formula?
Accurate to 3-5% for most runners between distances of similar type. It is most reliable when predicting adjacent distances (e.g. 10K → half marathon). Predicting a marathon from a 5K time introduces more error.
Which fatigue exponent should I use?
Use 1.04 for elite runners, 1.06 for recreational runners, and 1.08–1.10 for beginners or those undertrained for the goal distance. The default 1.06 works well for most people.