Race Time Predictor

Known distance, km

Hours

Minutes

Seconds

Target distance, km

Result

Predicted time3:27:01
Pace at the target4:54 min/km
Pace at the known distance4:30 min/km
Slowdown, %9.02%
Average speed, km/h12.23
What mattersRiegel's formula holds while the two distances are within a factor of four and the training matches the target.

This result is informational only and is not professional advice.

Time grows faster than distance

Predicts a time over a longer distance from a result at a shorter one, using Riegel's formula.

If pace held, a marathon would take exactly the four-and-a-bit times a 10K does. In practice it takes longer: pace inevitably falls as distance grows, and it falls predictably.

Pete Riegel described that with a power law and an exponent of 1.06. The formula is simple, has held up for half a century, and remains a working guide — provided the training matches the target distance.

What this calculator shows

The predicted time over the target distance
Pace at the target and pace at the known distance
The percentage slowdown between them

What to keep in mind

Riegel's formula holds while the distances are within a factor of four. Predicting a marathon from a 5K is already very optimistic.
It assumes training built for the target distance: without long runs, a real marathon comes in well behind the prediction.

FAQs

How does Riegel's formula work?

Time is multiplied by the ratio of the distances raised to the power 1.06. The exponent exceeds one, which is the inevitable loss of pace.

How accurate is a marathon prediction?

Within a few minutes on proper marathon training. Without long runs the real result is usually 10 to 20 minutes worse.

Which distance predicts best?

The half marathon: it is closest to a marathon in the demand it makes. Predictions from a 5K systematically overstate what is possible.

Does it work in reverse?

Yes, the formula is symmetric. From a marathon time it gives a 10K guide, though short distances depend more on raw speed.

Worked example

From a 10K to a marathon

Input: 10 km in 45:00, target 42.195 km

Output: 3:27:01 at 4:54 per km against 4:30

Note: A nine percent slowdown is the price of the distance. A runner who tries to hold their 10K pace is all but guaranteed to blow up past thirty kilometres.

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Race Time Predictor

Result

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