Predict your race time from a recent result
You ran 10K in 45:00. What does that say about a half marathon? The Riegel formula turns one result into equivalent times.
Tools used in this guide
The job
Training plans and race entries want a goal time. The Riegel formula — t₂ = t₁ × (d₂/d₁)^1.06 — converts a known performance into a prediction at another distance, with the exponent capturing the slowdown over longer runs.
One result in, one prediction out
Enter a recent distance and your finish time, then pick the target — 5K, 10K, half, full marathon, or a custom distance. The tool returns the predicted time and the pace it takes.
Worked example
45:00 for 10K predicts 21:35 for 5K, 1:39:17 for the half, and 3:27:01 for the marathon — a required 4:54/km pace, up from the 4:30/km input. The predictions are only as honest as the input — a flat-out 5K flatters your marathon number.
Scope
Distances 0.1–500 km, times 60 s–48 h, exponent fixed at 1.06. Real finishing times depend on training volume, course, and weather — treat the output as a target range, not a promise.