Riegel's formula: T2 = T1 × (D2/D1)^1.06. Predict any race time from a known performance.
A 25-minute 5K predicts a 2:03 marathon. A 45-minute 10K predicts a 1:35 half marathon.
The formula assumes consistent training and fitness level across both distances.
Use this calculator as a starting point for any health or fitness goal that requires a numeric benchmark. The result is an estimate, not a diagnosis — but it provides a concrete figure to track against over time.
A frequent error is failing to control measurement conditions between calculations. Weight and body circumferences vary significantly throughout the day and with hydration levels. Always measure under the same conditions (same time of day, same equipment) to make comparisons meaningful.
Someone returning from a period of inactivity uses the calculator to establish a baseline health metric before starting a new training programme. Monthly recalculations with consistent measurement conditions create a trackable trend that scale weight alone cannot provide.
Riegel's formula is accurate within 5–10 minutes for most trained runners. Accuracy decreases for untrained or very slow runners.
T2 = T1 × (D2/D1)^1.06, where T1 is your known race time, D1 the distance, and D2 the target distance.
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