Zerdly

Race Time Predictor

Predicts equivalent race times across distances using Riegel's formula.

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How to use this calculator

Enter details

Known distance(km)
Known time
::
Target distance(km)

Result

Fill in the fields to see your result.

Estimate only — real results depend on terrain, weather, fatigue, conditions and individual fitness. Not medical or coaching advice.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter a known race distance and the time you ran it in.
  2. Enter the target distance you want a prediction for.
  3. Read the predicted time and pace.

How it works

Uses Riegel's formula: t₂ = t₁ × (d₂ ÷ d₁)^1.06. It's most reliable when the two distances aren't too far apart and your training suits the longer race.

Worked example

10 km → marathon. A 40:00 10 km predicts roughly a 3:09 marathon (assuming marathon-specific training).

Common mistakes

  • Predicting a marathon from a 5k — the further the extrapolation, the larger the error.
  • Assuming the model accounts for training; it assumes you have trained for the longer distance.

Tips

  • Times are estimates only — not medical or coaching advice. Always train safely and follow event rules.
  • Distances are editable: choose “Custom…” to enter your own course lengths.

Frequently asked questions

How does the race time predictor work?

It uses Riegel's formula — new time = old time × (new distance ÷ old distance)^1.06 — so longer races are predicted at a slightly slower pace.

How accurate is it?

Good between nearby distances when you are trained for both; predicting a marathon from a 5k overestimates fitness without the endurance base.

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Cite this tool

Zerdly. (2026). Race Time Predictor. Retrieved from https://www.zerdly.com/tools/race-time-predictor/

Tip: Enter any known values to calculate the remaining results.

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