Race Time Predictor

Riegel is a good first approximation, not a guarantee — expect the marathon prediction to sit within a few minutes at best, and further out if your longest training runs are short. The original 1.06 exponent came from elite data and underpredicts marathon times for most runners, which is why this tool raises the exponent to 1.08 for marathon predictions. Recreational runners experience proportionally more fatigue at longer distances due to lower aerobic fitness and pacing errors, so treat any marathon predicted from a 5K as an optimistic ceiling rather than a target.

Peak distance running performance is maintained until approximately age 35–40, after which a gradual linear decline occurs. The decline accelerates after age 75–78. Age-grading tables normalize performances across ages, allowing fair comparisons. A 50-year-old running 3:20 may have a higher age-graded score than a 25-year-old running 3:05.

Yes, but with caveats. The Riegel formula provides an estimate, but marathon-specific factors (fueling, heat tolerance, training volume) create more variance at longer distances. A prediction from a half marathon result is more reliable than from a 5K. The further apart the distances, the wider the confidence interval.