Running / Race times
Race Time Predictor
Predict your 5K, 10K, half marathon, and marathon times from a race you have run, with Riegel's formula, and see the pace each would take per kilometre and per mile.
Race Time Predictor: Peter Riegel found that, for a trained runner, time grows a little faster than distance: T₂ = T₁ × (D₂ ÷ D₁)^1.06. Doubling the distance takes about 2.08 times as long, because the pace slows by roughly 4%. A 25-minute 5K predicts about 52 minutes for 10K and 3 hours 59 minutes for a marathon. Runs 100% locally in your browser with zero server file uploads.
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| Race | Predicted time | Pace per km | Pace per mile |
|---|---|---|---|
| 1500 m | 6:59 | 4:39 | 7:29 |
| 1 mile | 7:31 | 4:40 | 7:31 |
| 5K | 25:00 | 5:00 | 8:03 |
| 10K | 52:07 | 5:13 | 8:23 |
| 15K | 1:20:07 | 5:20 | 8:36 |
| 10 miles | 1:26:19 | 5:22 | 8:38 |
| Half marathon | 1:55:00 | 5:27 | 8:46 |
| Marathon | 3:59:47 | 5:41 | 9:09 |
Your pace: 5:00 per km over 5 km.
Riegel's formula, T₂ = T₁ × (D₂ ÷ D₁)^1.06, assumes you are equally trained for both distances. It is most reliable between races a few times apart in length; predictions for a marathon from a short race are often too fast unless you have done the long training runs. A larger exponent, such as 1.08, gives more cautious times.
A worked example
From a 5K in 25:00, a half marathon is 25 × (21.0975 ÷ 5)^1.06 = about 115 minutes, or 1:55:00, at 5:27 per kilometre against 5:00 in the 5K.
To train at the right intensity for those paces, see the heart rate zone calculator.
Where the formula comes from
Peter Riegel, an engineer, fitted world records in running, swimming, cycling, and walking in the 1970s and found the same shape in each: time grows with distance raised to a power a little above 1. For running he gave 1.06, for efforts of about 3.5 minutes to 4 hours.
How to use it
- Choose the race you have run, or enter another distance in kilometres.
- Enter your time, such as 25:00 or 1:52:30.
- Read the predicted time and pace for each distance; raise the exponent for more cautious predictions.
Privacy & limitations
Times are worked out in your browser and are not saved or sent anywhere.
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Frequently asked questions
How accurate is it?
For distances a few times apart and a runner trained for both, predictions are often within a few per cent. Marathon predictions from short races tend to be optimistic for runners without the long training.
What exponent should I use?
1.06 is Riegel's value. Runners who fade over longer distances often fit 1.07 to 1.08 better; using a recent race close to the target distance helps more than changing the exponent.
Does it account for hills or heat?
No: it assumes similar courses and conditions. A hilly or hot race will be slower than predicted.
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