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Race Time Predictor (Riegel)
Predict your race times across distances from a single recent result. This calculator uses the Riegel formula — T2 = T1 × (D2/D1)^1.06 — to project one race you have run out to the 5K, 10K, half-marathon and marathon. From a 25:00 5K it estimates a 10K of about 52:07, a half around 1:55 and a marathon near 3:59; a 20:00 5K points to roughly a 3:12 marathon. Enter your recent race distance and time in the predictor and all four projections appear at once, in km or miles. The honest caveat matters: Riegel is reliable from a mile up to the half-marathon and gets optimistic beyond it, and it is most accurate when your input race is at least half the target distance. The exponent shifts a little by level (≈ 1.04 advanced to 1.08 beginner). A predicted time assumes adequate training and endurance — treat it as a target, not a guarantee.
Riegel: T2 = T1 × (D2/D1)^1.06 across 5K / 10K / half / marathon
- Example: 25:00 5K → 10K 52:07, half ~1:55, marathon ~3:59
- Reliable to the half; optimistic beyond — best when input ≥ half the target
Frequently asked questions
How does the Riegel race predictor work?
It scales one race time to another distance with T2 = T1 × (D2/D1)^1.06, where T1 is your time over distance D1 and D2 is the target. The 1.06 exponent captures how pace slows as distance grows. Enter a recent race and it computes your predicted 5K, 10K, half-marathon and marathon times together.
Why is my predicted marathon time too fast?
Riegel is optimistic beyond the half-marathon because it assumes the same relative endurance at every distance, which rarely holds without marathon-specific training. Use a recent half rather than a 5K for the most realistic marathon estimate, and add a few minutes of buffer for a first marathon.