Running tools / Predictor
Race time predictor.
Estimate your marathon and half marathon times from a recent race, using Riegel's formula. It's a starting point for choosing a goal, not a guarantee.
Predicted from 5K in 25:00
Half marathon
1:55:00
5:27 /km · 8:46 /mile
Splits for this time →Marathon
3:59:47
5:41 /km · 9:09 /mile
Splits for this time →
An estimate, assuming you train for the distance. From a race this short, the marathon figure is likely to be optimistic.
Riegel's formula, exponent 1.06
Common predictions
| Recent race | Half marathon | Marathon |
|---|---|---|
| 5K in 20:00 | 1:32:00 | 3:11:49 |
| 5K in 25:00 | 1:55:00 | 3:59:47 |
| 10K in 40:00 | 1:28:15 | 3:04:00 |
| 10K in 45:00 | 1:39:17 | 3:27:01 |
| 10K in 50:00 | 1:50:19 | 3:50:01 |
| 10K in 55:00 | 2:01:21 | 4:13:01 |
| 10K in 1:00:00 | 2:12:23 | 4:36:01 |
| Half marathon in 1:30:00 | — | 3:07:39 |
| Half marathon in 1:45:00 | — | 3:38:55 |
| Half marathon in 2:00:00 | — | 4:10:12 |
How it works
Riegel's formula predicts a time T2 for distance D2 from a result T1 over D1: T2 = T1 × (D2 ÷ D1)^1.06. The exponent of 1.06 is a little more than 1 because pace slows as races get longer. A 1:45:00 half marathon predicts 3:38:55 for the marathon, a little more than double.
It's an estimate. The formula assumes you've done the training for the longer distance: the weekly mileage and long runs that a marathon needs. Most runners don't hold their shorter-race fitness all the way to 42 km, so predictions from a 5K or 10K are usually optimistic for the marathon. Use a recent half if you have one, and treat the prediction as the fastest you might run rather than the time to start at.
Pick a race for your goal
Questions
- How accurate is Riegel's formula?
- It's a good guide between neighbouring distances, such as a half marathon to a marathon, and less reliable the further apart they are. It assumes you've trained properly for the longer race: for runners whose training is built around shorter races, marathon predictions are usually too fast, often by several minutes or more. Treat it as a ceiling to pace sensibly under, not a promise.
- What marathon time does a 50-minute 10K predict?
- About 3:50:01, and about 1:50:19 for the half marathon. Many runners will be slower over the marathon unless their weekly mileage and long runs are marathon-ready.
- What marathon time does a 1:45 half marathon predict?
- About 3:38:55, at 5:11 per km. A recent half is the most useful single result for predicting a marathon.
- What is Riegel's formula?
- T2 = T1 × (D2 ÷ D1)^1.06. Peter Riegel published it in 1977 from race results across distances: the exponent of 1.06 means each doubling of distance takes a little more than twice as long, because you can't hold the same pace for longer.
- What affects race day besides fitness?
- Heat and humidity, hills, wind, and how well you pace and fuel. A hot race can cost many minutes over a marathon. A flat course in cool weather gives you the best chance of matching a prediction.