Race Time Predictor
Enter a recent race result and this predictor estimates your finish time at other distances using the Riegel endurance formula.
The Riegel formula estimates your race time as your known time multiplied by the distance ratio raised to the power 1.06 — a 25:00 5K, for example, projects to a 3:59:47 marathon.
How the predictor works
This tool uses the Riegel formula, a simple endurance power law: your predicted time equals your known time multiplied by the distance ratio raised to the power 1.06. The exponent captures the fact that you cannot hold your 5K pace over a marathon — as distance grows, pace slows in a predictable way. Enter one recent, honestly-run race and it projects your times across the standard distances from 5K to the marathon.
The prediction is at its best when your input and target distances are close: a 10K from a 5K is far more trustworthy than a marathon from a 5K. It also assumes you are actually trained for the longer race — it tells you what is possible with the right endurance base, not what you will run off minimal long runs.
Getting the most from it
The classic 1.06 exponent was fitted to world records, so it can predict slightly-too-fast marathons for recreational runners; if your longer predictions feel optimistic, treat them as a ceiling and add a few minutes. Pacing discipline and fuelling matter enormously in the back half of a marathon, and the formula cannot see those. Use the result to set target paces, then plan sessions with the pace calculator and dial in effort with the target heart rate calculator.
Example predictions from a 25:00 5K
| Distance | Predicted time | Avg pace /km |
|---|---|---|
| 5K | 25:00 (given) | 5:00 |
| 10K | 52:07 | 5:13 |
| 10 mile | 1:26:19 | 5:22 |
| Half marathon | 1:55:00 | 5:27 |
| Marathon | 3:59:47 | 5:41 |
Riegel predictions (exponent 1.06) from a 25:00 5K, using the standard World Athletics road distances. Predictions assume equivalent training for each distance.
Riegel, "Athletic Records and Human Endurance," American Scientist (1981). Predictions are estimates, not guarantees.
Frequently asked questions
How accurate is the Riegel race time predictor?
It is a solid estimate, not a guarantee. Accuracy is best when the input and target distances are close — for example predicting a 10K from a 5K — and drops as the gap widens, with a marathon predicted from a 5K being the least reliable case. Because the classic exponent was fitted to elite performances, it tends to predict slightly too-fast marathons for everyday runners.
Why does the formula use 1.06, and should I change it?
1.06 is the fatigue exponent Peter Riegel fitted to world records from 100 m to 100 miles; it means each doubling of distance multiplies your time by a bit more than two. Because it comes from elite data, it tends to predict slightly-too-fast marathons for everyday runners, so if your longer predictions feel optimistic, add a few minutes and treat them as a ceiling.
What distances does the predictor work best for?
It is designed for the endurance range and works well across standard road distances from about 1 K up to the marathon. It is most reliable when your input race and target race are within a reasonable range of each other. It is not designed for very short sprints or for trail and ultra events, where terrain and extreme fatigue break the simple power-law assumption.
Does the predictor assume I'm actually trained for the longer distance?
Yes. The formula assumes you have equivalent endurance and adequate, specific training for the target distance. Predicting a marathon time from a 5K tells you what is possible if your endurance base supports it — not what you will run off minimal long-run training. Pacing discipline and fuelling also matter, especially in the second half of a marathon.
Can I predict a shorter race from a longer one?
Yes — the formula works in both directions because it only depends on the distance ratio, so you can predict a 5K from a recent half marathon. That said, most runners are relatively stronger at either short or long distances, so a same-direction, close-distance prediction such as a 10K from a 5K will usually be the most trustworthy.
Sources
- Vickers & Vertosick — An empirical study of race times in recreational endurance runners (BMC Sports Sci Med Rehabil, 2016) — States Riegel formula and 1.06 exponent
- Blythe & Kiraly — Prediction and Quantification of Individual Athletic Performance of Runners (PLoS ONE, 2016) — Evaluates Riegel power-law race predictor
- Dash — Win Your Race Goal: A Generalized Approach to Prediction of Running Performance (Sports Med Int Open, 2024) — Benchmarks Riegel formula across race distances
- World Athletics — Marathon — Official marathon distance 42.195 km
The thresholds and formulas used by this calculator follow the primary sources above. This tool provides general information, not personalised medical advice.