Marathon Time Predictor
Predict your marathon finish time from one or two recent race results, using the Riegel formula with the Vickers-Vertosick marathon correction
Inputs
H:MM:SS or MM:SS
Miles in a normal training week — a fitted term in the model, not optional
Results
Why this is slower than a plain Riegel estimate
Riegel alone predicts 3:53:21, which is 12:50 faster.
Vickers and Vertosick tested Riegel against 2,303 recreational runners and found it gives marathon times at least 10 minutes too fast for half of them, while staying well calibrated up to the half-marathon. The correction above is that finding applied to your inputs — it is not a safety margin we invented.
Shorter distances (Riegel — uncorrected, and correctly so)
How marathon-time prediction works
The classic Riegel formula (Pete Riegel, 1981) predicts your time over a new distance from a recent race time:
T₂ = T₁ × (D₂ / D₁)^k
The exponent k is a fatigue factor: how much you slow down as the distance grows. Riegel quoted 1.08 for elite runners and 1.05–1.06 for recreational runners; this tool uses 1.07, the value Vickers and Vertosick adopted as an average.
Riegel is reliable — until the marathon. Vickers and Vertosick (2016) surveyed 2,303 recreational runners and tested Riegel against their real results. Up to the half-marathon it was well calibrated. At the marathon it was not: it predicted times at least 10 minutes too fast for half of all runners. Mean squared error on their validation set was 381 for Riegel against 228 and 208 for their two replacement models.
So this calculator does two different things. For the half-marathon, 10K and 5K it uses Riegel unchanged, because the evidence says Riegel is fine there. For the marathon it uses the Vickers-Vertosick model, which folds in your typical weekly training mileage — and, if you give a second race, the fatigue slope measured between your own two results rather than a population average. That second model is the more accurate of the two.
Riegel PS. Athletic records and human endurance. American Scientist. 1981;69(3):285-290. · Vickers AJ, Vertosick EA. An empirical study of race times in recreational endurance runners. BMC Sports Sci Med Rehabil. 2016;8(1):26. PMID 27570626 · doi:10.1186/s13102-016-0052-y