Half marathon time predictor.
A shorter race, a realistic goal.

Enter a recent 5K or 10K. See what it projects for the half marathon distance, using Jack Daniels’ VDOT.

Free tool · Updated April 19, 2026

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Your half marathon

Recent race distance

Finish time

Predicted half marathon finish

1:41:58

VDOT 44. Prediction assumes similar training and a flat course. Real races bring weather, hills, and how you slept the night before.

Other distances at this fitness

  • 10K46:00
  • Marathon3:31:44

Prediction is the number. Reshape builds the weeks that turn it into a finish line.

Why a 10K predicts a half well

A 10K runs at roughly 90% of VO2max; a half marathon at roughly 85%. That’s a small step on the percent-of-VO2max curve, which is why the prediction is usually tight. If you’ve run a sharp 10K recently, the half prediction is a number you can plan around.

5K as an input works, but it’s farther from half-marathon intensity, so the model extrapolates more. If you can swap in a 10K, do. If not, treat the number as a target to train toward.

What closes the gap between prediction and race day: a few 90-minute long runs, one or two HM-pace blocks in the last three weeks, and landing on the start line rested. Reshape builds that last month so the model becomes reality.

Common questions

It’s closer than a 5K-to-marathon jump, but still an extrapolation. A 10K is the best short-distance input for a half. If you only have a 5K, the number is a ceiling to train toward.

Use a hard time trial on a flat route — a 5K effort you couldn’t hold a conversation through. That’s close enough for the prediction to be useful.

Build the long-run backbone — 90 to 120 minutes of steady running, with a few half-marathon-pace blocks in the last month. Without that, you’ll race fitter than you trained, and the last 5K will feel like a different sport.

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