How to predict your race time

If you know your time over one distance, you can estimate your times over others with the Riegel formula: T2 equals T1 multiplied by the ratio of the distances raised to the power 1.06. That 1.06 exponent captures how pace naturally slows as distance grows. A recent 10K is the most reliable input for predicting a half or marathon. The predictions assume you have trained for the target distance and pace evenly, so treat longer races, especially the marathon, as a best case rather than a guarantee.

The formula coaches use

Pete Riegel published his endurance model in 1981, and race calculators have used it ever since. It says your predicted time over a new distance equals your known time, scaled by the distance ratio raised to the power 1.06. If you simply assumed the same pace at every distance the exponent would be 1.0; the extra 0.06 is the slowdown that every runner feels as the race gets longer.

The race time predictor applies this across 5K, 10K, half marathon and marathon from a single result, and shows the per-kilometre pace each one needs.

Pick a good input

• Use a recent, hard effort, ideally within the last few weeks.
• The closer the input distance is to your target, the better. A 10K predicts a half well; a parkrun 5K predicts a marathon only loosely.
• A flat, accurately measured course gives a cleaner number than a hilly or short one.

Where it breaks down

Riegel assumes matched training and even pacing. If you have done all your running at 5K but want to race a marathon, the model will flatter you, because it cannot see that you lack the endurance base. The marathon also has a wall that no formula predicts: glycogen depletion and pacing errors can add many minutes. Use the prediction to set a target band and a starting pace, not a finishing time set in stone.

Put it in context

A typical UK adult parkrun 5K lands somewhere around 28 to 32 minutes depending on age and sex, per public parkrun results. You can see more running benchmarks on the UK fitness statistics page, and run your own numbers in the calculators.

For information only. Build distance gradually and train for the event you are targeting.