Running Pace Predictor
Predict race times from a known distance and time using the Riegel formula. 5K to marathon predictions.
Quick Answer
The Riegel formula predicts race times: T2 = T1 × (D2/D1)^1.06. A 25-minute 5K runner can expect approximately a 52-minute 10K, a 1:55 half marathon, and a 4:02 marathon. The exponent 1.06 accounts for fatigue over longer distances.
Your Known Result
Predicted Race Times
About This Tool
The Running Pace Predictor uses the Riegel formula, the most widely cited race prediction model in running, to estimate your finish time at various distances based on a recent race result. Peter Riegel published this formula in 1977 in Runner’s World, and it remains remarkably accurate decades later for trained runners racing common distances from 5K to the marathon.
How the Riegel Formula Works
The formula T2 = T1 times (D2/D1) raised to the power 1.06 predicts that as race distance increases, your average pace slows in a predictable way. The exponent 1.06 captures the fatigue factor: doubling the distance does not double the time but increases it by about 6% more than doubling. This accounts for glycogen depletion, muscle fatigue, and the aerobic demands of longer efforts. The formula works best when your known time is from a recent, all-out race effort.
Accuracy and Limitations
The Riegel formula is most accurate when predicting between adjacent distances (5K to 10K, or half to full marathon). Predicting a marathon from a 5K time is less reliable because the physiological demands are so different. The formula also assumes equal training and race-day conditions. A well-trained marathoner will beat their Riegel prediction, while someone who trains only short distances will likely run slower than predicted at the marathon. Weather, course elevation, and nutrition strategy all affect marathon performance more than shorter races.
Using Predictions for Training
Race predictions are useful for setting realistic goal paces during training. If your predicted marathon time is 4:00, your long run pace should be around 9:45-10:30 per mile, and your tempo runs around 8:30-8:45 per mile. Many training plans use predicted race times to set workout paces for intervals, tempo runs, and easy days. Jack Daniels and other coaches have built entire training systems around race-equivalent paces.