Race time predictor
Enter a recent race result and the distance you want to run to get a predicted finish time and pace.
How the calculation works
Peter Riegel noticed that the world records at different distances follow a smooth curve: every time the distance doubles, the pace slows by about 6%. That gives one formula for every distance.
The result shows your predicted time, the pace it needs, and a table with the prediction at every standard distance from the same starting point.
T2 = T1 × (D2 ÷ D1)1.06
- T1
- your time over the known distance
- D1
- the known distance
- D2
- the distance you want to predict
- T2
- the predicted time
Predicted times from a 5K
These are the times the formula gives for a 5K result. Use the closest row as a quick check, then enter your own result for exact figures.
| 5K time | 10K | Half marathon | Marathon |
|---|---|---|---|
| 20:00 | 41:42 | 1:32:00 | 3:11:49 |
| 22:30 | 46:55 | 1:43:30 | 3:35:48 |
| 25:00 | 52:07 | 1:55:00 | 3:59:47 |
| 30:00 | 1:02:33 | 2:18:00 | 4:47:44 |
What the prediction cannot know
The formula assumes you are as well prepared for the new distance as for the one you ran. A marathon prediction from a 5K ignores that a marathon also needs long runs, fuelling and pacing discipline, so it is usually optimistic. Hills, heat, wind and a bad day all add time. Treat the result as a realistic upper bound for a well-trained runner, and set a slightly more conservative goal for race day.
Examples
Frequently asked questions
What is the Riegel formula?
A formula from 1977 that predicts a race time at one distance from a result at another. The new time is the old time multiplied by the ratio of the distances raised to the power 1.06.
How accurate is the prediction?
It is a good guide between roughly 5 km and the marathon, for runners who train for the distance they predict. It does not know your training, the course, the weather or how you feel on the day.
Why is my marathon prediction faster than I can run?
A fast 5K or 10K needs speed; a marathon needs endurance. The formula assumes you have trained for both. If your longest runs are short, expect a slower marathon than predicted.
Which race should I use as the starting point?
Use a recent race of 5 km or more, run on a flat course in good conditions. The closer it is to the distance you want to predict, the better.
Related calculators
Sources
- Riegel, P. S. (1981). Athletic Records and Human Endurance. American Scientist, 69(3), 285–290.
- World Athletics, Technical Rules – standard road race distances (the marathon is 42.195 km, the half marathon 21.0975 km)