Race Time Predictor

If you have run one race, you can make a reasonable guess at another. The standard tool for this is Riegel's formula, published after the author analysed a large set of running performances: your time at a new distance scales with the ratio of the distances raised to the power of about 1.06. The exponent above one is the interesting part. It says that time grows slightly faster than distance, which is exactly what fatigue does — doubling the distance costs you more than double the time.

Put numbers to it. A 50-minute 10K predicts a half marathon of roughly 1:50 and a marathon of about 3:50. Those are eminently sensible, and they explain why the marathon is not merely "two half marathons stuck together". The 1.06 exponent has held up well across thousands of runners, though the exact best-fit value varies a little by distance and by the individual.

The formula assumes you have trained for the target distance. That assumption is where it breaks. If your longest run is 12 kilometres, the marathon prediction is a piece of arithmetic rather than a forecast — most runners fade badly in the last ten kilometres of a debut marathon because glycogen runs out and the muscles are not yet adapted to the pounding. Use the prediction to set a starting pace, not to set a goal you have not trained for.

A few caveats worth carrying. Short, sharp efforts such as a 1500 metre track race predict poorly, because speed and endurance are different qualities and the exponent shifts at short distances. A race run in unusually good weather or on a downhill course will predict times you cannot reproduce. And heat, hills and a bad night's sleep all cost more time over longer distances. Treat the outputs as a fair estimate for a comparable course in comparable conditions, and as a reason to train rather than to book flights.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.