Running pace and race prediction
If you know your time over one distance, you can estimate your times over others. FitCalcs uses the Riegel formula, the standard endurance model, to predict your 5K, 10K, half marathon and marathon from a single recent result, along with the per-kilometre pace each one needs. Longer-distance predictions assume you have trained for that distance and pace evenly, so treat the marathon estimate as a best case rather than a promise.
Race time predictor
How this is worked out
Predictions use Riegel's endurance formula, the model running coaches and race calculators have used since 1981:
T2 = T1 × (D2 ÷ D1)^1.06
T1 is your known time over distance D1; the 1.06 exponent reflects how pace slows as distance grows. It assumes equivalent training and even pacing, so longer races (especially the marathon) are best-case estimates, not guarantees.
For information only. Build distance gradually and train for the event you are targeting.
More running and training calculators
Calculators and Data Desk, FitCalcs
FitCalcs' editorial desk builds and documents the calculators, citing the underlying equation and the UK dataset behind every number. Health-related tools are editorially reviewed, with figures cited to named UK sources.
Last reviewed: 12 June 2026