Race Time Predictor
Predict your finish time for any race distance using the Riegel formula.
Results
Wondering what time you could run a marathon based on your 10K result? The Riegel formula is the most widely used race prediction model, relied on by coaches and running apps worldwide. It works by applying a fatigue factor (exponent 1.06) to account for the fact that you slow down over longer distances. Enter any recent race time and get a prediction for your target distance along with the pace you would need to hold.
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Frequently asked questions
The Riegel formula is accurate within about 3-5% for most trained runners predicting between adjacent distances (e.g. 10K to half marathon). It becomes less reliable for very large jumps like 5K to marathon.
T2 = T1 x (D2/D1)^1.06, where T1 is your known time, D1 is the known distance, D2 is the target distance and 1.06 is the fatigue factor. It was published by Pete Riegel in 1977.
The 1.06 exponent accounts for fatigue - your body slows down as distance increases due to glycogen depletion, muscle fatigue and accumulated impact. A 50-minute 10K runner does not simply run a 1:40 half marathon.
Use a race time from the last 2-3 months where you ran at full effort. Training runs and easy-paced races will give overly slow predictions. A proper race effort gives the most accurate input.
The Riegel formula underestimates ultra times because fatigue compounds differently beyond marathon distance. For ultras, specialized models that factor in elevation, terrain and walking breaks are more appropriate.