Table of Contents
How Race Prediction Works
Race prediction uses mathematical models to estimate how long it will take you to run a race distance you have not yet attempted, based on your performance at a known distance. The most widely used model is the Riegel formula, published by Peter Riegel in 1977 and still considered remarkably accurate for distances from 1,500 meters to the marathon.
The core insight behind race prediction is that pace naturally slows as distance increases due to physiological fatigue. The relationship between time and distance follows a power law: doubling the distance does not simply double the time. The exponent in the Riegel formula (1.06) captures this fatigue factor for the average trained runner.
The Riegel Formula
Where T1 is the known time, D1 is the known distance, D2 is the target distance, and T2 is the predicted time. The exponent 1.06 represents the average fatigue factor. Values below 1.06 indicate above-average endurance; values above indicate below-average endurance.
Prediction Examples
| Known Race | Known Time | Predicted Half Marathon | Predicted Marathon |
|---|---|---|---|
| 5K | 20:00 | 1:31:36 | 3:11:47 |
| 5K | 25:00 | 1:54:30 | 3:59:44 |
| 10K | 45:00 | 1:39:06 | 3:27:23 |
| 10K | 55:00 | 2:01:07 | 4:13:28 |
Frequently Asked Questions
How accurate is the Riegel formula?
For well-trained runners racing distances between 5K and the marathon, the Riegel formula is typically accurate within 2-5%. Accuracy decreases for undertrained runners, ultramarathon distances, and when predicting a much longer race from a short one (e.g., 5K to marathon).
Why is my marathon slower than predicted?
The formula assumes proportional training. If you train primarily for shorter distances, you may lack the long-run endurance needed for a marathon. Nutrition, hydration, weather, and pacing strategy also significantly affect marathon performance beyond what the formula can predict.
Should I use my fastest or most recent race?
Use your most recent race that reflects your current fitness. A PR from years ago may not represent your current ability. Ideally, use a race run within the last 2-3 months at a similar effort level to your target race.