Last updated: 3 October 2026
Reviewed by Jordan Morley, FA Level 2 Football Coach
Race time prediction uses a recent performance to forecast what you could run over a different distance. The most common method is Riegel's formula, which scales your time by the ratio of distances raised to a fatigue exponent (about 1.06). Predictions are most accurate when you've trained specifically for the target distance — a 5K time tends to over-predict marathon ability unless you've built the endurance base.
- Riegel's formula is the common method
- Most accurate for trained distances
- Longer predictions assume endurance training
- Useful for setting realistic race goals
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Race time prediction models generally work best when the known performance and target race are reasonably close in both distance and the specific endurance demands involved — a recent 10k time predicts a marathon time less reliably than it predicts a half-marathon, since the marathon depends much more heavily on long-run-specific endurance and fuelling that a 10k performance doesn't fully capture.
FAQ
Why isn't a 10k time a reliable predictor of marathon performance?
The marathon depends heavily on long-duration endurance, fuelling strategy and fatigue resistance that a much shorter 10k effort doesn't adequately test, making the prediction gap wider and less reliable the further apart the distances are.
What additional factors affect race time predictions beyond the formula?
Training specificity for the target distance, pacing strategy, weather conditions on race day, and course terrain all meaningfully affect actual performance beyond what a pure time-based formula can predict.