Racing · Free tool
Race Time Predictor
Enter a recent race result — distance and time — and choose a target distance. We use Pete Riegel’s endurance model to predict an equivalent performance.
How the Riegel formula works
The predictor uses T₂ = T₁ × (D₂ ÷ D₁)^1.06. The exponent 1.06 captures the fact that pace fades as distance grows: you cannot hold your 5K pace for a marathon. Feed it a recent, honest result and it returns the time an equivalently-fit runner would post at the new distance.
It is most accurate when the two distances are reasonably close and the effort lasts between about three minutes and four hours. Predicting a marathon from a 5K stretches the model — the further apart the distances, the more your endurance base (not just your speed) decides the outcome.
Use it as a ceiling, not a promise
A prediction assumes you have done the specific training for the target distance. A fast 5K runner who has never run beyond 10 km will not hit their predicted marathon time without months of long runs. Treat the number as the performance your current speed makes possible, conditional on the endurance work to back it up.
Frequently asked questions
Why is my predicted marathon time too fast?
The Riegel model tends to be optimistic for the marathon because it cannot see your endurance base. If your long-run training is limited, expect to finish slower than the prediction and adjust your goal accordingly.
Which recent race should I use?
Use your most recent all-out effort at a distance close to your target. A 10K or half-marathon result predicts a marathon far better than a parkrun 5K does.