Race Time Predictor - Riegel Formula Calculator
Run a great 10K and wondering what you can run in a marathon? This race time predictor uses the proven Riegel Formula to calculate your potential times across all distances - 5K, 10K, half marathon, and marathon. Enter one recent race result and get science-based predictions for your next race. Perfect for setting realistic goals and planning training paces. Free, trusted by coaches worldwide. It helps you move from guesswork to a confident decision with clear inputs and readable output. Adjust values, compare scenarios, and share results quickly. Runs client-side in your browser, with no signup required. Built for speed, clarity, and repeat use. Method details for Race Time Predictor Calculator: The result model exposes each formula and equation, applies deterministic calculation steps, uses explicit decimal rounding, and keeps unit assumptions visible so outputs are auditable.
Predicted Race Times
📊 About Riegel's Formula
Predictions are based on T₂ = T₁ × (D₂/D₁)^1.06. Results are estimates and actual performance varies based on training, conditions, and race strategy. Use as a training guide, not a guarantee.
How to Predict Your Race Times
- Enter a recent race time - Your best 5K, 10K, half, or full marathon
- Select the race distance - What distance was that time for?
- View predictions - See equivalent times for all other distances
- Adjust for training - Predictions assume proportional training volume
Understanding the Riegel Formula
The Riegel Formula is: T2 = T1 × (D2 / D1)^1.06 where T is time, D is distance, and the exponent 1.06 represents the fatigue factor. This formula, developed by marathoner Pete Riegel in 1977, accounts for the fact that you can't maintain the same pace as distance increases.
Why 1.06? The exponent means that doubling distance more than doubles time. For example, if you run a 45-minute 10K (7:15/mile pace), the formula predicts a 3:19 marathon - not at 7:15/mile (3:09), but at 7:37/mile accounting for endurance degradation.
Pro tip: This formula is most accurate for runners with balanced training. If you only train for 5Ks, your marathon prediction will be optimistic. Conversely, high-mileage marathoners often outperform their 5K-based predictions. Use this as a baseline, then adjust based on your training focus.