Race Time Predictor

You ran a solid 5K. What does that mean for your half marathon? The race time predictor above takes a recent result and projects your finish time at another distance, using the formula distance runners have leaned on for decades.

How the prediction works

The calculator uses Riegel's model, which says that as race distance doubles, your time more than doubles by a small, predictable amount. It is a power relationship with an endurance exponent baked in. Plug in a recent race distance and time, pick your target distance, and you get a projected finish and the pace it implies.

The model is remarkably good across nearby distances. A 5K predicting a 10K, or a 10K predicting a half, tends to land close. The further the jump, the more assumptions it makes.

The big caveat

A prediction is not a promise. Riegel assumes you are properly trained for the longer distance. A fast 5K runner who has never run beyond 10K will not hit the predicted marathon time, because the marathon is limited by endurance and fueling, not just speed. The formula cannot see your long-run volume, your fueling practice, or how your legs hold up after two hours.

Use the prediction as a ceiling, the time you could run if your endurance matched your speed. Then train the gap. If the predicted marathon looks far quicker than feels realistic, that gap is your training plan: more long runs, more time on feet, practiced race fueling.

Using it to pace, not just predict

The most practical use is pacing. The predicted finish gives you a target average pace, which you can then run as even or slightly negative splits. Going out faster than your predicted pace is the single most common way to blow up late, especially in the half and the marathon.

For how to anchor that pacing to effort rather than just the clock, especially over the marathon where heart rate keeps you honest, read the marathon heart rate guide.