Race Time Predictor
Predict your race times based on recent performance
What is a Race Time Predictor?
A race time predictor estimates how fast you can run a target distance based on a known recent performance at another distance. If you recently ran a 10K in 50 minutes, the predictor can estimate your likely marathon finish time — saving you from the costly mistake of starting a marathon at an unrealistic pace.
This calculator uses the Riegel formula, developed by Peter Riegel and published in American Scientist (1981). The formula is: T2 = T1 × (D2/D1)^1.06. The exponent 1.06 captures the well-established fact that performance degrades at longer distances — a runner who is 10% faster over 5K than another runner will not be 10% faster over a marathon. The fatigue effect compounds with distance.
The Riegel formula is the most widely used prediction model in running, used by coaches, race organisers, and running apps worldwide. Its primary limitation is that it assumes similar training preparation for both distances. A 5K specialist who has not done long runs will underperform the prediction at marathon distance.
How to Use This Calculator
Typical Race Equivalency (Average Club Runner)
- • 5K in 25:00: → 10K ~52:00 | HM ~1:55 | Marathon ~4:02
- • 10K in 50:00: → HM ~1:51 | Marathon ~3:54
- • 10K in 45:00: → HM ~1:39 | Marathon ~3:30
- • HM in 1:45: → Marathon ~3:41
- • HM in 2:00: → Marathon ~4:13
?Frequently Asked Questions
How accurate is the Riegel formula?
For predictions between similar distances (e.g. 5K to 10K, or 10K to half marathon), the Riegel formula is typically accurate within 2–5% for well-trained runners. Accuracy decreases significantly when predicting across very different distances — e.g. 5K to marathon — because the physiological demands are fundamentally different. A 5K relies heavily on VO2max, while a marathon is primarily a fat metabolism and pacing challenge.
Why does the formula use 1.06 as the exponent?
The 1.06 exponent was derived by Peter Riegel from analysis of world record performances across distances, published in American Scientist (1981). It represents the average rate at which human performance degrades with increasing distance. Some researchers have proposed alternative exponents (Cameron 1985 used distance-specific exponents), but 1.06 remains the standard for its simplicity and reasonable accuracy.
Should I use a 5K or 10K time to predict my marathon?
A 10K or half marathon time gives more accurate marathon predictions than a 5K, because the physiological systems (aerobic capacity, fat metabolism, muscular endurance) involved in a 10K or half marathon are more similar to marathon demands. Using a 5K time will typically give an optimistic marathon prediction unless you have specifically trained for marathon endurance. A recent half marathon is the gold standard input for marathon prediction.
What is the 10% rule for increasing training volume?
The 10% rule — not increasing weekly mileage by more than 10% per week — is a widely used guideline to reduce injury risk. It was popularised by running coaches in the 1980s and is referenced in most recreational running programmes. Research from the British Journal of Sports Medicine (2014) supports gradual progression, though the exact percentage is debated. Most coaches recommend 2–3 easy weeks for every 3–4 weeks of progression.