Race time predictor (Riegel)
Enter a recent race time and the distance you are training for, and the tool estimates your finish time and pace.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter known race distance, other known distance, hours, minutes, seconds and target distance.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Peter Riegel formula (1977): T2 = T1 × (D2 ÷ D1)^1.06, where T1 is the known race time, D1 its distance and D2 the target distance. The exponent is editable.
Example
Inputs: Known race distance = Half marathon (21.0975 km), Hours = 1, Minutes = 45, Seconds = 0, Target distance = Marathon (42.195 km). Result: Predicted time: 3:38:55; Predicted pace per kilometer: 5:11 min/km; Predicted pace per mile: 8:21 min/mile.
Limits and caveats
- The formula assumes you have trained properly for the target distance, so marathon predictions are often optimistic for low mileage runners.
- It is most reliable for efforts between about 3.5 minutes and 4 hours.
Questions about this tool
What is the Riegel formula?
A formula published by engineer Peter Riegel that scales race time by distance to the power 1.06, so pace slows a little as distance grows.
If I run a 50 minute 10K, what marathon time can I expect?
The formula predicts about 3:50:01, a pace of 5:27 per kilometer.
Why change the fatigue exponent?
Runners with strong endurance may fit an exponent below 1.06, while runners who fade over long distances may fit a higher one.