Race Time Predictor
Predict a finish time for any distance from a recent result using Riegel's proven formula.
Estimated with Riegel: t2 = t1 x (d2 / d1) ^ 1.06. Treat it as a best-case ceiling that assumes equal training for both distances.
The race time predictor turns a result you already have into an estimate for a race you have not run yet. Enter a recent distance and time, choose the distance you are targeting, and it projects a finish time along with the pace you would need to hold. It is the quickest way to set a sensible goal, sanity-check an ambitious target, or compare efforts across different distances on a level footing.
Riegel's formula explained
The prediction uses Peter Riegel's endurance formula, published in the 1970s and still a standard reference. It reads t2 = t1 x (d2 / d1) ^ 1.06, where t1 and d1 are your known time and distance, d2 is the target distance, and 1.06 is the fatigue exponent. The exponent is the important part. If it were exactly 1, doubling the distance would simply double the time. Because it is slightly above 1, each step up in distance costs a little extra per kilometre, which matches how real runners fade as races get longer. For example, a 20:00 5K predicts a marathon of roughly 3:11, not the 2:48 you would get by naive doubling.
Getting a trustworthy estimate
Two things drive accuracy: how recent your input race is, and how close the two distances are. A prediction from 10K to half marathon is usually within a few percent for a trained runner. Jumping from a 5K straight to a marathon stretches the model, because the marathon rewards endurance, pacing discipline and fuelling that a 5K never tests. Treat the number as a best-case ceiling that assumes you have trained specifically for the target distance.
From prediction to race day
Once you have a target time, translate it into a race plan. Feed the pace into the splits calculator to map out each kilometre, or check the relevant pace chart, such as the half marathon pace chart or marathon pace chart, to see exactly what your watch should show. The main pace calculator is handy for converting the predicted pace between units.
Frequently asked questions
What is Riegel's formula?
Riegel's formula predicts a finish time for one distance from a result at another: t2 = t1 x (d2 / d1) raised to the power 1.06. The 1.06 exponent captures the natural slowdown as distance grows.
How accurate is a race time prediction?
It is a solid estimate when the two distances are reasonably close and you are trained for both. Predicting a marathon from a 5K tends to be optimistic because endurance and fuelling matter more over 42 km.
What race result should I use?
Use a recent, honest, all-out effort. A 10K from last month predicts a half marathon far better than a 5K from two years ago. The closer the distances, the more reliable the number.
Why does a longer race need a slower pace?
You can hold a higher power output for a short race than a long one. Riegel models this: doubling the distance adds roughly 6 percent to the per-kilometre time rather than simply doubling the finish.