Enter one recent race result to predict your times at other distances.
| Distance | Predicted time | Pace /km | Pace /mi |
|---|
Enter a recent finish time above to see predictions.
This predictor uses the Riegel formula, published by engineer Pete Riegel in 1981 and still the most widely used race prediction model: T2 = T1 × (D2 ÷ D1)1.06. The exponent 1.06 captures how much runners slow down as the distance grows — roughly 6% more time per doubling of distance beyond simple proportionality. It has held up remarkably well across four decades of race data for distances from 1500 m to the marathon.
For example, a 25:00 5K predicts a 52:07 10K — not 50:00, because nobody holds their 5K pace for twice the distance. The same 25:00 5K predicts roughly a 1:55 half marathon and a 4:00 marathon.
For well-trained runners predicting adjacent distances (5K→10K, 10K→half), it's usually within 1–2%. Marathon predictions are less reliable because they depend heavily on endurance training — many runners run 10–20 minutes slower than their 5K-predicted marathon time if their weekly mileage is low.
The formula assumes your endurance scales with your speed. If you haven't built marathon-specific mileage (long runs of 28 km+ and consistent 50 km+ weeks), expect to be slower than predicted. Use the prediction as a training target, not a race-day pace plan.
Your most recent all-out effort at a shorter distance — ideally an official race, or a solo time trial where you genuinely emptied the tank. The fresher and more honest the input, the better the prediction.
Yes — divide the predicted time by the distance to get target pace (shown in the table). Many runners aim for the predicted pace through 80% of the race and speed up at the end if they feel strong.
VDOT (Jack Daniels) and Cameron's model give similar results for most runners. Riegel is the simplest and most transparent. For training zones rather than race predictions, use our VDOT-based Training Zone Calculator.