Enter your recent 5K 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 Scientistin 1981. It assumes that performance degrades predictably with distance — faster runners degrade slightly less, slower runners slightly more. The formula works best when your 5K was a genuine race effort, and when your aerobic base is proportional to your speed. If you're a strong 5K runner but have never run more than 10 km, your actual HM time will likely be slower than the prediction.
The predictor uses the Riegel formula (T2 = T1 × (D2/D1)^1.06), a widely validated race prediction model. It's most accurate when your 5K was raced at full effort and was run recently. Expect ±3–5 minutes for well-trained runners; larger gaps if your aerobic endurance doesn't match your speed.
Ideally within the last 3–6 months. An older time or a non-race effort will still give a useful ballpark, but the prediction will be less precise. Use a parkrun result or a recent time trial for best accuracy.
Based on the Riegel formula, a 5K around 27:30 or faster suggests a sub-2-hour half marathon is achievable — assuming your long-run endurance is developed. Speed alone isn't enough; consistent weekly mileage matters just as much.
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