Race-time predictor: what are you capable of right now?
Enter a result from any distance and the Riegel formula will estimate your potential times and paces for 5K, 10K, half marathon and marathon. You can share the prediction by link.
Select a distance and enter your time; predictions will appear automatically.
How the Riegel formula works
Peter Riegel introduced the empirical relationship T₂ = T₁ × (D₂/D₁)^1.06 in 1977. The 1.06 exponent represents the cost of distance: doubling the distance typically slows pace by about 6%. The formula remains a standard after half a century and underpins VDOT tables and many watch calculators.
Its limitation is the assumption that you are equally well prepared for both distances. An amateur running 30 km per week may receive an optimistic marathon prediction from a strong 10K because they lack the necessary volume. Predictions are generally more reliable when the source and target distances are closer.
How to get a more accurate prediction
- Use a race result rather than a training run: race effort is different. A recent competitive 10K is a strong starting point.
- Avoid jumping across several distances: marathon predictions from 5K are the least reliable. A robust progression is 5K → 10K → half marathon → marathon.
- Account for course profile and weather: the formula knows nothing about a hill at 32 km or 25 °C heat. Hilly courses and hot conditions can add 2–5% to your time.
- Adjust amateur marathon predictions: if weekly volume is below 50–60 km, adding 5–10 minutes to the Riegel estimate is often prudent.
What to do with the prediction
A prediction is a target time, not a training plan. Achieving it requires aerobic base training, threshold work, race-specific sessions and an appropriate taper. See our guides on easy volume, lactate and thresholds and an example training week.
Monitor load progression too: abrupt volume spikes can undermine fitness. Check your ACWR and use the heart-rate zone calculator to establish training intensities.
FAQ
How accurate is the Riegel prediction?
For neighbouring distances such as 10K to half marathon, a trained runner may see an error of around 1–3%. Accuracy declines as distances move farther apart and running experience decreases. Amateur marathon predictions from a 5K can be optimistic by 5–15 minutes.
Should I enter chip time or watch time?
Use official chip time when available, measured from when you cross the start line to the finish. Watch time is reasonable if you start near the front and lose little time in congestion. A training result can be used, but account for roughly 3–5% from race-day motivation and competition.
Why is the exponent 1.06?
It is an empirical value derived from large numbers of race results. Elite runners may be closer to 1.05 because they are well prepared across distances; amateurs may be 1.08 or higher. We use the classic 1.06 as a reasonable, moderately optimistic benchmark.
Does it work for cross-country skiing, cycling and swimming?
Formally yes, but the appropriate exponent differs. Aerodynamic drag has a strong effect in cycling, while technique has an outsized influence in swimming. The calculator is most reliable for running and for cross-country skiing on similar terrain.
See how it works with your own data
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