Free marathon time calculator and race predictor. Estimate 5K, 10K, half marathon, and marathon finish times from a recent race using Riegel's formula.
This race predictor estimates how you might perform at a different distance based on a previous race or time trial. Enter the distance and finish time from a recent effort, then choose your target race distance — for example, predict a half marathon from a 50-minute 10K — to see a projected finish time.
The calculation uses Riegel's formula (T₂ = T₁ × (D₂/D₁)^1.06), a well-known model in endurance running that accounts for the fact that pace slows as race distance increases. It is a useful starting point for setting goals and comparing performances across distances, such as projecting a 10K time from a 25-minute 5K.
Predictions are generally more reliable for shorter events such as predicting a 5K from a 10K result, predicting a 10K from a 5K result, and predicting a half marathon from a 10K result. Marathon predictions from a half marathon time tend to be optimistic, because the formula assumes you can sustain a similar effort curve over much longer distances — something that usually requires dedicated marathon training and pacing experience.
Treat these projections as a guide rather than a guarantee. Course profile, weather, training load, and race-day conditions all affect how closely your actual result will match the prediction.
Other tools that pair well with this calculator.
Free running pace calculator for km and miles. Enter distance, finish time, or target pace to work out the missing value, with kilometre splits.
Estimate VO2 max from a recent race distance and finish time. A practical fitness benchmark for runners tracking aerobic progress.
Convert running distances between kilometres and miles. Quickly switch 5K, 10K, half marathon, and marathon distances between metric and imperial.
