Race Time Predictor

This calculator predicts your race time at one distance from a result you have already run at another, using Riegel's formula, the most widely used model for relating running performances across distances. Runners constantly want to know what their current fitness means for a goal race: if you can run 5 kilometres in a certain time, what does that suggest for a 10 kilometre, a half marathon or a full marathon? Pacing a race too fast off an unrealistic target is one of the most common ways to blow up, so a sensible prediction helps you set goal times and even splits you can actually hold. Riegel's formula captures the well-known fact that pace slows as distance grows, scaling your time by the ratio of the distances raised to a fatigue factor of about 1.06. You enter a recent race distance and the time you ran it in, then the distance you want to predict, and the calculator returns the estimated time for the target distance, along with the pace per kilometre for both the known and predicted runs so you can see how much your pace is expected to slow. Use it to set a realistic goal time, to plan race-day pacing, and to track how a faster 5 kilometre time should translate to your longer races. The prediction is most reliable when the two distances are not wildly different and when you have trained appropriately for the longer distance, since endurance and fuelling matter enormously in the marathon. Treat it as a well-grounded guide that assumes similar conditions and specific training, not a guarantee, and always start your race conservatively.

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Riegel's formula (fatigue factor 1.06). Most reliable when distances are similar and you are trained for the longer one. A guide; start conservatively.