Health Calculators
Race time predictor.
Enter a recent race result and predict your time at 5K, 10K, 15K, 10 miles, the half marathon and the marathon with the Riegel formula.
Your recent race
Predicted times
Fill the form and press Predict.
How the Riegel formula works
Riegel's power-law equation is T2 = T1 × (D2 / D1)k: the time at a new distance equals your known time times the distance ratio raised to a fatigue exponent k. Riegel's 1981 paper used world records and gave k = 1.06 as the standard; the 2016 study of recreational runners by Vickers and Vertosick reports Riegel citing 1.08 for elite runners and 1.05 to 1.06 for male recreational runners aged 40 to 70, and used 1.07 as an average.
Marathon caution: that study found the formula well-calibrated up to a half marathon but too optimistic for the marathon, giving times at least 10 minutes too fast for about half of the runners. Treat a marathon prediction from a 5K as a best case, and prefer a longer source race when you have one.
Sources: Riegel PS, "Athletic Records and Human Endurance," American Scientist 69(3):285-290, 1981 (PubMed). Vickers AJ, Vertosick EA, "An empirical study of race times in recreational endurance runners," BMC Sports Science, Medicine and Rehabilitation 8:26, 2016 (PMC5000509).
Want the pace behind a predicted time? Use the running pace calculator.
Disclaimer
A prediction assumes equal training, course and conditions at both distances. It is an estimate, not a guarantee. For personalized training, work with a qualified running coach.