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Race Time Predictor

Enter one recent race result and get predicted times for 5K, 10K, half and full marathon — using the Riegel formula runners actually trust.

Written by Jude Ramsey · Last updated August 2026 · How these calculators are checked

How one race predicts another

Pete Riegel’s 1977 formula is still the standard: multiply your time by the distance ratio raised to the power 1.06 — the exponent that captures how pace naturally slows as distance grows. Enter any recent race and this calculator projects the other classic distances, on the honest assumption that you’d train appropriately for each.

How to use it

  1. Pick the race you’ve actually run recently.
  2. Enter the finishing time.
  3. Read the predictions for the other distances.

The formula

T₂ = T₁ × (D₂ ÷ D₁)^1.06

A worked example

A 25:00 5K projects to 52:07 for 10K, 1:55:00 for a half marathon and just under 4:00 for the full — note it is not simple doubling: the 10K prediction is 2:07 slower than twice the 5K, and that gap is the 1.06 exponent at work. The marathon number is the least reliable of the set: without the long-run mileage, most runners fall short of it, which says more about training than about the formula.

Common questions

Why is my marathon slower than predicted?
Riegel assumes equivalent training for the target distance — and marathon endurance is built on weekly mileage most 5K runners don’t carry. The prediction is the ceiling your speed supports, not a promise.
Which race predicts best?
The closest one — a 10K predicts a half far better than a 5K predicts a marathon. Recent beats old, and race efforts beat training runs.
Is 1.06 right for everyone?
It’s the population-fit average — speed-leaning runners fade slightly more (higher exponent), endurance monsters slightly less. Your own two race results can back out your personal exponent.

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