How the prediction works
The model is Pete Riegel's: finish time scales with distance raised to the power 1.06. A 1:35 half marathon therefore predicts about 3:18 for a marathon, because 42.195 km is twice 21.1 km and 2^1.06 is 2.08. The exponent captures the fact that longer races are disproportionately harder.
Why the prediction can be wrong
Riegel's formula assumes you have trained for the longer distance. Someone who races a fast 10K off 30 km a week will not run the marathon the formula suggests; someone with three 30 km long runs will. For most runners the honest adjustment is 3–8 minutes slower than the raw number, and more if the course is hilly or the day is warm.
Using it well
Pick a race from the last three months, run on a comparable course, and treat the output as a range rather than a target. The equivalent-times table is useful for checking consistency: if your 5K predicts 3:05 but your half predicts 3:25, endurance is the limiter, not speed.
Frequently Asked Questions
How accurate is Riegel's formula?
Within 2–5% for runners with appropriate mileage. It over-predicts for 5K specialists moving up and under-predicts for high-mileage runners.
Can I use a training run instead of a race?
Only if it was an all-out effort on a measured course. A comfortable long run will predict a marathon far slower than you can actually run.
Why does the exponent matter?
A larger exponent makes longer races slower. 1.06 is the standard; 1.07 is more conservative and often closer for first marathons.
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