Running Pace Calculator
Enter any two of distance, time and pace — the third is calculated. The prediction table uses Riegel’s endurance exponent to estimate what a result at one distance implies at others.
Your Run
Fill in distance plus either time or pace; the missing one is solved.
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Pace and speed are the same fact, inverted
Pace is time per unit distance — minutes per kilometre. Speed is distance per unit time — kilometres per hour. Runners think in pace because it maps directly onto what a watch shows mid-run; cyclists think in speed. One is simply 60 divided by the other.
The Riegel formula, and where it breaks
Pete Riegel’s 1977 model predicts a time at a new distance from a known
result: T₂ = T₁ × (D₂ / D₁)1.06.
The exponent above 1 encodes the fact that you cannot hold short-race pace over
long distances.
It is accurate within a factor of about three to four in distance. Predicting a marathon from a 5K stretches it well beyond that, and the estimate assumes marathon-specific endurance you may not have trained. Treat a Riegel marathon prediction from a short race as a ceiling, not a plan.
Even splits versus the reality of racing
This calculator assumes a constant pace. Almost no one races that way: most positive-split, running the second half slower. The exception is a well-executed marathon, where a slight negative split is the classic pattern for a good day. Use even-pace numbers as the target and expect drift.