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Marathon Time Calculator

Estimate your time for marathon, half marathon, 10K, and 5K from a reference time using Riegel's formula.

Sources: Riegel P. -- Athletic Records and Human Endurance, American Scientist 1981 · World Athletics -- Race Predictions

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How the Marathon Time Calculator Works

This tool estimates your time for marathon, half marathon, 10K, and 5K from a reference time, using Riegel's formula, the most validated predictive model in running science.

Riegel's Formula

Published in 1981 in American Scientist: T2 = T1 x (D2 / D1) ^ 1.06

The exponent 1.06 means that doubling the distance increases time by more than double (about 6% extra), reflecting the physiological limits of the human body in endurance events.

Practical Example

If you run 10K in 50 minutes:

  • 5K estimated: ~23:40 (pace 4:44)
  • Half marathon estimated: ~1h49 (pace 5:10)
  • Marathon estimated: ~3h51 (pace 5:28)

Limitations

The formula is more accurate when the reference time is recent, you have trained specifically for the target distance, and the starting distance is reasonably close to the target. For the marathon, the estimate from 10K tends to be optimistic for less experienced runners.

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Frequently Asked Questions

How do you estimate marathon time?▼
The most used method is Riegel's formula: T2 = T1 x (D2/D1)^1.06. Starting from your 10K time, the formula accounts for the natural slowdown over longer distances (exponent 1.06). For example, with 50 minutes on the 10K, the marathon estimate is about 3h51.
How accurate is the estimate?▼
Riegel's formula is accurate for runners who train specifically for the target distance. For the half marathon, the error is typically 2-3%. For the marathon, the error can reach 5-8% because energy management and specific training have greater weight.
Why is marathon pace slower than 10K pace?▼
Over long distances, the body fatigues and pace naturally slows. Riegel's formula models this with the 1.06 exponent: doubling the distance, the time more than doubles. A 5:00 min/km runner on the 10K will run the marathon at about 5:30-5:40 min/km.