Improve Your Running With a Personalized Plan
Work with CM Performance Coaching for a custom training plan built around your goals, schedule, and experience level.
Get Your Custom Plan →
Enter a recent race result and predict your finish time at any distance using the Riegel formula.
Improve Your Running With a Personalized Plan
Work with CM Performance Coaching for a custom training plan built around your goals, schedule, and experience level.
Get Your Custom Plan →The race time predictor uses your recent race result to forecast a finish time at any distance — from 5K to marathon. Once you have your predicted time, use the Running Split Calculator to plan your race splits, or check the Marathon Pace Chart if you're targeting 26.2.
This calculator uses the Riegel formula, developed by Pete Riegel and published in Runner's World in 1977. It's the most widely-used mathematical model for predicting race performance across distances.
The formula is: T2 = T1 × (D2 ÷ D1) ^ 1.06
In plain English: take your recent finish time, multiply it by the ratio of the new distance to your previous distance, then raise that ratio to the power of 1.06. The exponent accounts for the fact that longer races require a slightly slower pace than shorter ones — fatigue compounds over distance.
For example, if you ran a 5K in 25:00, the formula predicts your marathon time as: 1500 × (42.195 ÷ 5) ^ 1.06 ≈ 3:59:42. It's a starting point — individual factors like training, course profile, and conditions all affect actual race day performance.
Common 5K finish times and their predicted race times at each distance, calculated using the Riegel formula.
| 5K Time | 10K | Half Marathon | Marathon |
|---|---|---|---|
| 18:00 | 37:31 | 1:22:49 | 2:52:35 |
| 20:00 | 41:42 | 1:32:01 | 3:11:45 |
| 22:00 | 45:52 | 1:41:13 | 3:30:56 |
| 25:00 | 52:07 | 1:55:01 | 3:59:42 |
| 28:00 | 58:22 | 2:08:49 | 4:28:28 |
| 30:00 | 1:02:33 | 2:18:01 | 4:47:38 |
| 35:00 | 1:12:58 | 2:41:02 | 5:35:35 |
The Riegel formula is a solid baseline, but it works best when predicting between distances that are reasonably close — such as 5K to 10K, or 10K to half marathon. Predictions become less accurate at extreme distances (e.g., using a 5K time to predict a 100-mile ultramarathon). It also assumes both races were run at full effort and under similar conditions.
Use the predicted time as a training target and adjust based on your specific race course, weather, and fitness.