Race Time Predictor
Predict 5K, 10K, half and marathon times from one race with Riegel's formula.
About this tool
Pick the race you ran, enter your time as h:mm:ss or mm:ss, and the table predicts times for the mile, 5K, 10K, 15K, 10 miles, half marathon and marathon, with pace per kilometre and per mile. Add a custom distance in kilometres, or raise the fatigue exponent for a more cautious prediction.
How race times are predicted
In 1977 engineer Peter Riegel noticed that endurance performance follows a simple power law: Tโ = Tโ ร (Dโ รท Dโ)^1.06. The exponent above 1 captures how you slow down as the race gets longer. A 50-minute 10K predicts about a 23:59 5K, a 1:50:20 half marathon and a 3:50 marathon. The prediction is most reliable between similar distances and when the source race was recent, all-out and on a fair course. Longer extrapolations, especially to the marathon, are optimistic for many runners because the marathon also depends on fuelling and weekly mileage. Raising the exponent to 1.07 or 1.08 gives a more conservative target. Honest limits: heat, hills, wind, altitude and how well you've trained for the distance all matter, so treat predictions as a pacing guide. The pace calculator converts the result into split times, the VO2 max calculator estimates fitness and the heart rate zones calculator sets training intensities. Running pace explained: converting pace, speed and race times, and what pace to train at explains pace and speed.
Frequently asked questions
How accurate is Riegel's formula?
It's usually within a few percent between similar distances, such as 10K to half marathon, but often optimistic for the marathon.
What marathon time does a 2-hour half marathon predict?
About 4:10 with the standard 1.06 exponent; many runners should plan for a few minutes slower.
What is the fatigue exponent?
The power in Riegel's formula. 1.06 is standard; higher values predict slower times at longer distances.
Is my data saved?
No. The calculation runs in your browser and nothing is stored or sent.