Predict any race finish time from a known result
Your Known Race Performance
Pace: 4:42 /km • 7:34 /mi
Target Race
Predicted Finish Time
All Distances — Predicted Times
| Distance | Predicted Time |
|---|---|
| 400 m | 1:37 |
| 800 m | 3:22 |
| 1 km | 4:16 |
| 1 mile | 7:04 |
| 1500 m | 6:34 |
| 3 km | 13:40 |
| 10 km | 48:60 |
| 15 km | 1:15:18 |
| Half Marathon | 1:48:06 |
| 25 km | 2:09:25 |
| 30 km | 2:37:00 |
| Marathon | 3:45:23 |
| 50 km | 4:29:49 |
| 100 km | 9:22:33 |
How It Works — Riegel Formula
Developed by Pete Riegel and published in Runner's World (1977), this formula predicts how long it will take to cover a new distance based on a known race result. The exponent 1.06 reflects the natural slowdown effect as distance increases — longer races demand more energy relative to their length. It is widely used by coaches and runners to set realistic goals.
The formula is most accurate for distances between 1.5 km and 50 km and assumes consistent training and comparable race conditions. Ultra distances or very short sprints may produce less accurate results.
Note: Predictions are mathematical estimates based on the Riegel formula and are not a substitute for structured training guidance. Factors such as course elevation, weather, rest, and fitness changes can all affect actual race performance.
The Race Time Predictor uses the Riegel formula — T2 = T1 × (D2/D1)^1.06 — to estimate your finish time at any race distance from a single known result. Whether you are targeting a first marathon or a 10K personal best, enter your recent 5K, half marathon, or any custom distance time and get an instant, science-backed prediction with full pace breakdowns.
The Riegel formula (T2 = T1 × (D2/D1)^1.06) was published by Pete Riegel in 1977. It uses an exponent of 1.06 to model the natural fatigue effect: the longer the race, the slower your pace relative to shorter distances. It remains one of the most widely cited race prediction methods in endurance sports.
The formula is most accurate for distances between 1.5 km and 50 km when the comparison race was run at a genuine effort under similar conditions. Factors like course elevation, heat, wind, and current fitness level can cause real results to deviate by a few percent. Ultra-marathon distances (100 km+) tend to be underestimated.
Yes, but predictions will be less reliable. Race times reflect maximum effort, while training runs are usually paced conservatively. Using a time trial or tempo effort closer to race pace will give more realistic predictions.
The 1.06 exponent accounts for the compounding fatigue of longer distances. Most recreational runners actually slow down more than the formula predicts in a marathon due to fueling and training volume — so the Riegel estimate is often considered an optimistic ceiling rather than a guaranteed outcome.