Race Time Predictor

Predict your finish time at a different race distance using Riegel's formula — enter a known time at one distance (5K to marathon) to estimate your time at another.

Predicted times from common results

If you ran	5K	10K	Half marathon	Marathon
5K in 25:00	25:00	52:07	1:55:00	3:59:47
5K in 30:00	30:00	1:02:33	2:18:00	4:47:44
10K in 50:00	23:59	50:00	1:50:19	3:50:01
Half in 2:00:00	26:05	54:23	2:00:00	4:10:12

Notice that doubling the distance more than doubles the time. The 1.06 exponent in Riegel's formula adds about 4% per doubling to account for fatigue.

Getting the most accurate prediction

• Use a recent, all-out race. A time from a race or time trial in the last 4–6 weeks is far more useful than an easy training run.
• Predict from the closest distance. A half marathon predicts a marathon better than a 5K does.
• Train for the distance. The formula assumes you've done the training for the longer race. Most first-time marathoners run slower than predicted, because a 5K or 10K doesn't test endurance the same way.
• Allow for the course and weather. Hills, heat, humidity and wind can add minutes to any prediction.

Using the result

A predicted time gives you a realistic goal and a starting race pace. Divide the predicted time by the distance to set your pace — a 3:59:47 marathon is about 9:09 per mile — then use the running pace calculator to plan your splits.

Frequently asked questions

What formula does this use?

Riegel's formula (Peter Riegel, 1977): T2 = T1 × (D2 / D1)^1.06 — a widely used model that accounts for the fact that pace naturally slows over longer distances due to fatigue.

How accurate is the prediction?

It's a solid estimate for well-trained runners at similar distances, but gets less precise the bigger the gap between your known distance and the target (e.g. predicting a marathon from a 5K is less reliable than from a half marathon).

What marathon time does a 25-minute 5K predict?

Riegel's formula predicts about 3:59:47, just under four hours. In practice, many runners need strong long-run training to match the prediction, so treat it as a best-case goal.

Why does my pace slow at longer distances?

Longer races use more of your energy stores and cause more muscle fatigue, so the pace you can hold drops as distance rises. Riegel's 1.06 exponent models that slowdown.