Race Time Equivalents Calculator
Enter one race result and see your equivalent 5K, 10K, half-marathon and marathon times using the Riegel endurance model.
How it works
T2 = T1 × (D2 ÷ D1)^1.06 (Riegel, fatigue exponent 1.06)
Peter Riegel’s 1977 model says finish time scales with distance raised to the power 1.06 — the exponent above 1 captures how pace slows as races get longer. Feed it one honest result and it predicts the others, assuming similar fitness and appropriate endurance training. Predictions for distances far from the one you ran (a marathon from a 5K) drift, because they assume marathon-specific endurance you may not have trained.
Worked example
A 50:00 10K gives 3000 s over 10 km. For the marathon: 3000 × (42.195 ÷ 10)^1.06 = 3000 × 4.60 ≈ 13,800 s = 3:50:00. For 5K: 3000 × (5 ÷ 10)^1.06 ≈ 1,439 s = 23:59.
Frequently asked questions
How accurate is the Riegel formula?
It is very good between nearby distances (5K↔10K, 10K↔half) for a well-trained runner. It tends to be optimistic for the marathon predicted from short races, because 26.2 miles depends heavily on fuelling and long-run endurance the model cannot see.
Why the exponent 1.06 and not 1.0?
If pace never slowed, time would scale linearly (exponent 1.0). Real runners fatigue, so doubling the distance more than doubles the time — Riegel fit 1.06 to race data as the average slow-down.
What race should I enter for the best marathon prediction?
Use your most recent half-marathon or a hard 10K. The closer the input distance is to your target, the smaller the extrapolation error.
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This calculator is for educational purposes. Double-check important results before acting on them.