Race Time Predictor
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
Enter details
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.
How to use this calculator
- Enter a known race distance and the time you ran it in.
- Enter the target distance you want a prediction for.
- 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).
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 accurate is it?
It's an estimate. Endurance, fuelling and specific training for the target distance all matter — treat it as a goal-setting guide.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.