Race Time Predictor Calculator (Riegel Formula)

The Riegel formula, published by Peter Riegel in American Scientist in 1981, predicts race time as: T2 = T1 x (D2 / D1)^1.06. T1 is your known time for distance D1, and D2 is the distance you want to predict. The exponent 1.06 accounts for the fact that pace gets slower as race distance increases (the fatigue factor).

The Riegel formula is reasonably accurate for distances ranging from 1.5 km to marathon distance. Accuracy decreases for ultramarathon distances because the fatigue exponent (1.06) was derived from data up to marathon distance. Individual factors such as training specificity, heat, hills, and race strategy also affect real-world accuracy.

Yes, but with reduced accuracy over large distance gaps. The formula will give a ballpark target. More reliable predictions come from using a race at a closer distance (for example, a half marathon to predict marathon time). As a rough check: a common rule of thumb is marathon time is approximately 2.1 times your half marathon time.

An exponent of exactly 1 would predict that pace stays perfectly constant across distances. In practice, longer races are run at slower paces due to physiological fatigue, glycogen depletion, and pacing strategy. Riegel found empirically that 1.06 best fitted observed race performances across a wide range of distances and athletes.

You can use any consistent units for distance (km, miles, or meters) as long as D1 and D2 use the same unit. Time can be entered in total minutes. The calculator handles the conversion automatically. For best results, use a recent race time from a well-paced effort, not a training run.