Enter your recent 10K finish time and see your predicted half marathon time, goal pace, and full training zones — instantly, no sign-up required.
Predicted times using the Riegel formula. For a personalised estimate, use the calculator above.
The calculator uses the Riegel formula (T₂ = T₁ × (D₂/D₁)^1.06), a race prediction model developed by Peter Riegel and published in American Scientist in 1981. Because a 10K is roughly half the half marathon distance, predictions from 10K times are generally more reliable than those from 5K times. The formula assumes your aerobic endurance scales proportionally with your speed — which holds well for runners who regularly run 40–60 km per week, but less well for runners whose long runs rarely exceed 10 km.
The predictor uses the Riegel formula (T2 = T1 × (D2/D1)^1.06), a widely validated race prediction model. Because a 10K is closer to a half marathon in distance than a 5K is, predictions from 10K times tend to be more accurate — typically within ±2–4 minutes for trained runners with a solid aerobic base.
Ideally within the last 3–6 months. A time from a well-paced race gives the most accurate prediction. A time trial or a parkrun double-lap works too. Avoid times from a heavily hilly course or a training run — both will skew the prediction.
Based on the Riegel formula, a 10K around 54:00 or faster suggests a sub-2-hour half marathon is achievable — provided your long-run endurance matches your speed. Runners who only run shorter distances often find the half marathon humbles them. Build your weekly long run to at least 15–16 km before race day.
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