Race time predictor (Riegel formula)
One recent race in, predictions for the mile, 5K, 10K, half and marathon out, with honest warnings.
How the math works
The predictor uses Peter Riegel's formula: T2 = T1 × (D2 ÷ D1)1.06. T1 and D1 are your known race, D2 is the target distance. Doubling the distance multiplies the time by 21.06 = 2.085, so your predicted pace is about 4% slower when the race gets twice as long.
Riegel described this curve in Runner's World in 1977 and in American Scientist in 1981, for efforts lasting roughly 3.5 to 230 minutes. In the 1981 paper, the fitted exponent for world-class men's running was 1.077. The rounded 1.06 is the version that spread among coaches and calculators.
The formula assumes you are equally trained for both distances. That is rarely true for a marathon predicted from a 5K: without the long runs, the real time is usually slower. Rows that stretch your race four times or more, or fall outside the 3.5 to 230 minute range, are flagged. You can raise the exponent to 1.07 or 1.08 for a more conservative prediction.
Estimates for planning. Weather, hills, course and fitness on the day all change the result. Formulas and distances reviewed October 21, 2026.