Race Time Predictor
Predict a race time from a result at another distance.
Predicted finishing time
1:39:17
pace 4:42 /km
Predictions between neighbouring distances are usually accurate within a couple of percent for a trained runner.
How the Race Time Predictor works
Riegel's formula scales a known race result to another distance using a power law fitted to real race data. It is remarkably good between neighbouring distances and progressively more optimistic as the gap widens — particularly when predicting a marathon from a 5K.
Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.
Frequently asked questions
What is Riegel's formula?
T2 = T1 × (D2 ÷ D1)^1.06. The 1.06 exponent captures how much pace decays as distance grows; it was fitted to large sets of race results in 1977 and has held up well since.
How accurate is race time prediction?
Good within adjacent distances — 5K to 10K, half to full — usually within a couple of percent for a trained runner. Predicting a marathon from a 5K is unreliable, because the marathon depends heavily on endurance-specific training and fuelling that a 5K result says nothing about.
Why is my marathon slower than predicted?
Almost always because the prediction assumes you trained specifically for the distance. Glycogen depletion, pacing errors and heat affect a marathon far more than a 10K. Many runners find an exponent of 1.10-1.15 fits their own marathon better than Riegel's 1.06.
Can I use this to set a goal pace?
As an upper bound, yes. Treat the prediction as what you might achieve on a good day with distance-appropriate training behind you, then plan to start slightly slower than it and negative split.