Free running calculator
Race time predictor
Predict a race time and target pace from a recent performance using the Riegel model, with clear warnings about long-distance extrapolation.
Reviewed December 11, 2025
Predicted finish
52:07
Target pace / km
5:13
Target pace / mile
8:23
What this result means
The Riegel model projects how finish time changes with distance using a fatigue exponent. It works best from a recent, hard effort on similar terrain and for reasonably close distances; marathon predictions from a short race are often too optimistic without matching endurance preparation.
Predicted time = known time × (target distance ÷ known distance)^1.06.
Choose the right known result
Use a recent race or hard time trial that reflects current fitness. A training run with stops or an easy effort will understate what you can race.
Predictions are more credible between nearby distances. A 10K can inform a half marathon more directly than a one-mile time can inform a marathon.
Why the marathon is different
Research in recreational runners found the classic model reasonably useful through the half marathon but poor for marathon prediction. Long events amplify the effects of fueling, pacing, durability, heat, and specific long-run preparation.
Treat the output as one pacing input. Recent long efforts and sustainable weekly volume should decide whether the predicted pace is plausible.
Frequently asked questions
How accurate is a race time predictor?
It is an estimate. Accuracy improves with a recent hard result at a similar distance and similar course conditions.
Can a 5K predict a marathon?
Mathematically yes, but the result can be too optimistic when marathon-specific endurance and fueling are not established.
What is the Riegel formula?
It scales a known finish time by the ratio of target to known distance raised to a fatigue exponent, commonly 1.06.
Sources and assumptions
Prediction meets preparation
Give your target pace a real training history.
LogFast tracks runs, effort, distance, and weekly structure together so its coach can test whether a calculated race scenario fits the work you have done.
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