Calculyn

Race Time Predictor

Predict your finish time for a longer race based on a race you've already run.

Results update as you type.

Predicted finish time

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Predictions are estimates for training purposes, not guarantees of performance.

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Quick answer

A 25:00 5K predicts a 10K of about 52:00 and a half marathon of roughly 1:55.

How it works

Uses the Riegel formula, the standard race prediction method: your time scales with distance raised to the power 1.06, which accounts for slowing down over longer distances.

Formula

New time = Old time × (New distance ÷ Old distance)^1.06

Assumptions & limitations

  • Riegel formula with 1.06 exponent
  • Adequate training for the target distance
  • Similar course and weather conditions

Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators

Frequently asked questions

How accurate is this?

Usually within a few percent if you've trained for the longer distance. It assumes similar fitness and conditions.

Why is my predicted marathon slower than double my half?

The 1.06 exponent builds in the natural slowdown that comes with distance.