Race Time Predictor

Yes. Half marathon (21.0975 km) is one of the four target distances the predictor outputs alongside 5K, 10K, and the marathon. Enter any one recent race result and it projects your half marathon finish using Riegel's T2 = T1 × (D2/D1)^1.06. Predictions are most accurate (within about ±5%) when your input distance is within roughly 3× the half marathon, so a 10K result gives a more reliable half marathon estimate than a 5K.

Riegel's endurance model (1977): T2 = T1 × (D2/D1)^1.06. The 1.06 exponent captures the non-linear fatigue increase with longer distances — you slow down proportionally more as distance increases.

Within ±5% for well-trained runners predicting to distances within 3× their baseline. Accuracy drops for first-time marathoners extrapolating from a 5K, or for athletes who are significantly stronger at one distance.

Riegel's formula assumes consistent aerobic fitness across all distances. If you are undertrained for the marathon (short long runs, not enough weekly volume), your actual time will be slower than predicted. The formula models potential, not current race readiness.

You can, but add 5–8% to your training time to account for the difference between race effort and a typical training effort. Race results give the most accurate predictions because they represent maximum aerobic output.

Yes. All calculations are client-side. No data leaves your browser.