Race time predictor
Enter one recent race result and get equivalent times for the mile, 5K, 10K, half marathon and marathon, using Pete Riegel's well-known endurance formula.
The Riegel formula
In 1981 engineer Pete Riegel published an analysis of world records in American Scientist ("Athletic Records and Human Endurance") showing that running time grows slightly faster than distance. His model is T2 = T1 × (D2 ÷ D1)1.06. Double the distance and your time goes up by about 2.085×, not 2×, because you can't hold the same pace for twice as long.
Worked examples
| 5K time | 1 mile | 10K | Half marathon | Marathon |
|---|---|---|---|---|
| 20:00 | 6:01 | 41:42 | 1:32:00 | 3:11:49 |
| 25:00 | 7:31 | 52:07 | 1:55:00 | 3:59:47 |
| 30:00 | 9:01 | 1:02:33 | 2:18:00 | 4:47:44 |
| 35:00 | 10:31 | 1:12:58 | 2:41:00 | 5:35:41 |
When predictions miss
The formula assumes you're equally trained for both distances. It's usually close between neighbouring distances (5K to 10K). It tends to be optimistic for the marathon if your weekly mileage is low, because the marathon depends on endurance and fuelling that a 5K doesn't test. Use predictions for pacing and goal-setting, then adjust to how training goes.
FAQ
How accurate is the Riegel formula?
It is a reasonable estimate between similar distances for runners trained for both. For marathons, many runners with low mileage run slower than predicted, so treat long-distance predictions as a best case.
Which race should I enter?
Use your most recent all-out race or time trial on a measured course. Older or easy-effort times will under-predict what you can do.
Why 1.06?
Riegel found exponents of about 1.06–1.08 fitted record performances across endurance events. 1.06 is the version most widely used by runners.
Related
- Running pace calculator – pace from time, or time from pace
- Split calculator – even and negative splits per km or mile
- Training pace calculator – easy, tempo, interval and repetition paces from your 5K
- Treadmill pace converter – mph or km/h to min/mile and min/km
- Km to mile pace converter – min/km ↔ min/mile
- All pace charts – printable tables