CALCULATOR

Race Time Predictor

Predict times across distances from one recent result using the Riegel formula, with a note on why marathon predictions run fast.

Predicted times

–

1 mile–

5 km–

10 km–

Half marathon–

Marathon–

Your pace in that race–

The Riegel formula uses an exponent of 1.06, which assumes pace degrades predictably with distance. It holds up well between 5 km and the half marathon and tends to be optimistic for the marathon.

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Predicting race times across distances

The Riegel formula predicts a time at one distance from a known time at another, using the relationship T2 = T1 x (D2 / D1) raised to the power of 1.06. The exponent reflects the fact that pace degrades predictably as distance increases.

It holds up reasonably well between about 5 km and the half marathon for trained runners. Predictions become less reliable at the extremes: very short distances depend heavily on speed rather than endurance, and marathon predictions from short races are commonly optimistic.

The reason marathon predictions overshoot is that the marathon introduces factors the formula cannot see, principally glycogen depletion and the specific endurance built by long training runs. A runner with a fast 5 km and low weekly mileage will not run the predicted marathon.

Frequently asked questions

How accurate is the Riegel formula?

Reasonable between 5 km and half marathon for trained runners, and typically optimistic for the marathon.

Why is my predicted marathon time too fast?

The formula assumes equivalent endurance training for the target distance. Marathon performance depends heavily on long-run volume and fuelling.

What exponent does the formula use?

1.06 in the standard version. Some coaches use slightly higher values for longer distances.

Can I use it for other sports?

It was derived for running. Cycling and swimming have different physiological and drag relationships.

Sources

Riegel PS. Athletic Records and Human Endurance. American Scientist, 1981.
Vickers AJ, Vertosick EA. An empirical study of race times in recreational endurance runners. BMC Sports Science, Medicine and Rehabilitation, 2016.
National Institutes of Health, Office of Dietary Supplements. Nutrient fact sheets. ods.od.nih.gov
NHS. Nutrition and health guidance. nhs.uk
US Departments of Agriculture and Health and Human Services. Dietary Guidelines for Americans, 2025-2030. dietaryguidelines.gov

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General information, not medical advice. Consult a qualified professional before changing your diet, especially with a medical condition.