Race Time Predictor

It uses Pete Riegel's formula: target time equals your known time multiplied by the ratio of the two distances raised to the power 1.06. The 1.06 exponent means predicted pace slows a little as the distance gets longer, which matches how most runners fade over a race.

It is a solid estimate for distances within a few times of your known race, especially when you have trained for the target distance. It is most accurate between 5K and the half marathon. Treat the numbers as a goal to aim for, not a guarantee.

The model assumes you are equally well trained at every distance, which is rarely true. It tends to be optimistic for the marathon if your longest runs are short, and for very short sprints where speed, not endurance, dominates. Add a buffer for distances far from your known race.

Use your most recent all-out effort at a distance close to your target. A recent 10K predicts a half marathon better than an old 5K does. Fresh, hard race results give the most reliable equivalents.

Endurance events draw more on aerobic limits and fuel stores, so almost everyone runs a slower pace per kilometre over a marathon than over a 5K. The 1.06 exponent in the formula builds in that expected slowdown.

No. The prediction runs entirely in your browser, so your race distance and time never leave your device.