Race Time Predictor
Enter a recent race distance and your time. You get predicted times and paces for the other common distances.
This is an estimate for general information and is not medical advice. Predictions assume you train for the longer distance.
Worked examples
20 minute 5K
5 km in 20:00
10K 41:42, half marathon 1:32:00, marathon 3:11:49
1200 s x 2^1.06 and so on.
About Race Time Predictor
Not sure what time to aim for in your next race? A race time predictor uses a result you already have to estimate the others. This one uses the formula published by Pete Riegel in 1977: the new time equals your known time multiplied by the new distance divided by the old distance, raised to the power of 1.06. The exponent is a little above 1 because runners slow down as the distance grows.
Enter a recent race distance and your finishing time and you get predicted times and paces for a mile, 5K, 10K, 10 miles, half marathon and marathon. You can change the fatigue factor if you tend to fade more or less than average over long distances. A 20 minute 5K predicts a 10K of 41:42. Predictions assume matching training and conditions. This is an estimate and not medical advice.
How to use Race Time Predictor
- Pick the distance of a recent race, or enter your own.
- Enter your finishing time, for example 25:00.
- Leave the fatigue factor at 1.06, or change it if you know you slow more or less over distance.
- Read the predicted times and paces.
Common uses
- Setting a goal time for a half marathon
- Checking whether a marathon target is realistic
- Choosing a race pace from a recent 5K
- Comparing results across distances
Frequently asked questions
What is the Riegel formula?
It is T2 = T1 x (D2 / D1) ^ 1.06, where T1 is your time over distance D1 and T2 is the predicted time over D2. It comes from Pete Riegel’s analysis of record times.
How accurate is it for a marathon?
It is often optimistic when predicting a marathon from a 5K, because the marathon needs specific endurance training. It is most reliable for distances up to two or three times the race you entered.
What does the fatigue factor do?
It is the exponent in the formula. 1.06 is the standard. A higher figure such as 1.07 predicts more slowing over longer distances, and a lower one such as 1.05 predicts less.
Which race should I use as the input?
Use a recent, all-out race on a similar course. A training run gives a slower result than your true form. A race close in distance to your target gives the best prediction.
Is this medical advice?
No. It is an estimate and not medical advice. Build up distance gradually, and speak to your GP if you have a health concern before starting hard training.