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Tool No. 02

Race
Predictor

Enter a recent race result and get accurate predicted finish times for any other major distance. Powered by the Riegel formula — The gold standard used by serious runners for decades. See what your recent 10K means for your upcoming half or full marathon.

Your Recent Race
: : H : M : S
Enter your official chip time. For best results use a race from the last 8 weeks.
The Riegel formula uses an exponent to model how pace slows with distance. 1.06 works for most trained runners. Adjust if you consistently over or underperform at longer distances.
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Enter a race result
to see your predictions

Predicted Finish Times
About the Riegel Formula — T2 = T1 × (D2 ÷ D1)^1.06. Developed by Peter Riegel in 1977, it remains the most widely used model for predicting race times across distances. It assumes consistent training and race conditions — it tends to be optimistic for runners undertrained at longer distances and conservative for those with a strong endurance base.
How It Works
The Riegel
Formula

Developed by Peter Riegel in 1977 and published in Runner's World, the formula models how pace slows as distance increases. The math: T2 = T1 × (D2 ÷ D1)^1.06 — where T1 is your known time, D1 is that race's distance, and D2 is the distance you're predicting.

The exponent (1.06) is the fatigue factor — it captures the physiological reality that the longer you run, the more your pace degrades. Trained runners with strong aerobic bases can use 1.05; runners who fade more at longer distances should use 1.07.

The formula is most accurate when predicting across similar distances (10K → half, half → marathon). It assumes equivalent training, conditions, and taper. Use results from within the last 8 weeks for best accuracy.

Common Questions
The Riegel formula is highly accurate for trained runners predicting across similar distances — typically within 2–5%. It becomes less accurate the larger the distance gap (predicting a marathon from a 5K, for example) and for runners who are significantly undertrained at the longer distance. It assumes equivalent conditions: similar terrain, similar weather, and a proper taper for both races.
Using the Riegel formula at the standard 1.06 exponent: a 45-minute 10K predicts a half marathon of approximately 1:40. A 50-minute 10K predicts roughly 1:52. A 55-minute 10K predicts roughly 2:04. These are fitness-equivalent predictions — they assume you're properly trained for the half marathon distance, not just fit enough to run a fast 10K.
Yes, and this is one of the most reliable uses of the Riegel formula. A 1:52 half marathon predicts roughly a 3:55 marathon. Note that marathon-specific training matters enormously: the Riegel prediction assumes equivalent preparation for both distances — not just half marathon fitness applied to 26.2 miles.
The fatigue factor (exponent) models how pace naturally slows as distance increases. The standard value of 1.06 works for most trained runners. Use 1.07 if you consistently fade at longer distances or have limited endurance-specific training. Use 1.05 if you have a strong aerobic base and hold pace well over long efforts. Most runners should leave it at 1.06.
The most common reason is training specificity — your source race reflects fitness you haven't yet translated to the longer distance. A fast 10K built through track work won't fully transfer to a half if you haven't logged the long runs. Other factors: poor pacing, heat, hills, or nutrition issues on race day. The Riegel prediction is your ceiling with optimal preparation, not a guarantee.
Within 8 weeks is ideal. Fitness changes meaningfully over longer periods — a 5K from 6 months ago doesn't reflect your current shape if you've been training hard since then. For the most accurate prediction, use a result from a recent tune-up race or a well-executed time trial on a flat course.
Field Note
Finding Where You Lie in the Data — half marathon and marathon time percentiles by age and gender, so your predicted time has real context.
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