Race Time Predictor
Enter one race result and predict your equivalent times across all standard distances. Based on the widely used Riegel formula.
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How the Race Time Predictor works
The calculator uses the Riegel formula: T2 = T1 × (D2/D1)^1.06. The exponent 1.06 accounts for the fact that runners slow down disproportionately as distance increases - running economy and fatigue have a larger effect on longer races. Predictions are most accurate when the input and target distances are within 2× of each other.
Frequently asked questions
How accurate is the race time predictor?
The Riegel formula is reliable for comparing similar distances. It becomes less accurate when predicting a marathon from a 5K because the two races require quite different physiological demands. A 10K to half marathon prediction, or a half marathon to marathon prediction, is typically within 2–3 minutes for well-trained runners.
Why is my marathon prediction slower than I expected?
The formula applies a fatigue penalty that increases with distance. Many runners also slow down in the second half of a marathon if undertrained - the formula assumes consistent fitness across both distances. If you've specifically trained for the marathon, you may outperform the prediction.
Can I use this to set a race goal?
Yes, with caution. Use it as a starting point, not a target. Also check the Training Paces Calculator to see if the target pace sits within a realistic training zone for your current fitness.