How race times are predicted
This is Peter Riegel's formula, published in 1981 from world-record data. The exponent 1.06 captures how pace slows as races get longer. It's the formula behind most race equivalency charts.
Example: a 25:00 5K predicts a 10K in 52:07, a half marathon in 1:55:00 and a marathon in 3:59:47, at about 9:09 per mile.
How accurate are the predictions?
They work best between nearby distances, such as 10K to half marathon, and when you've trained for the longer race. Predicting a marathon from a 5K tends to be optimistic for most recreational runners, because the marathon depends on endurance and fueling, not just speed. If your weekly mileage is low, raise the fatigue factor to 1.08 or 1.10 for long races; very well-trained runners may run close to 1.05.
Use a recent race on a similar course, run all-out. Hills, heat and wind all slow you down, so adjust your goal for race-day conditions. To turn a goal time into a pace, use the pace calculator.
Frequently asked questions
What marathon time does a 25-minute 5K predict?
About 3:59:47 with Riegel's formula, or about 9:09 per mile. Without marathon-specific training, expect to run somewhat slower.
How do I predict my half marathon time?
Enter a recent 5K or 10K result. A 50-minute 10K predicts a half marathon of about 1:50:19.
What is the fatigue factor?
The exponent in Riegel's formula, 1.06 by default. A higher number means you slow down more as the distance grows, which suits runners with less endurance training.
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