Marathon time predictor.
A shorter race, a realistic goal.

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

Free tool · Updated April 19, 2026

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Your marathon

Recent race distance

Finish time

Predicted marathon finish

3:37:50

VDOT 43. 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

  • Half marathon1:45:00
  • 10K47:22

Prediction is the ceiling a shorter race gives you. The marathon is the training that earns it.

Why shorter races predict longer ones

Jack Daniels noticed that race performance across distances follows a predictable curve: the percentage of VO2max you can hold falls as the distance grows. A 5K is run near the top of your aerobic ceiling. A marathon sits closer to 80% of it. If you know one point on the curve, the rest is math.

The closer your input race is to marathon effort, the smaller the extrapolation. Half marathon is the best input. 10K is solid. 5K is useful but optimistic — the marathon asks you to sustain an intensity for roughly 8x longer, and that requires training the model doesn’t see.

Treat the prediction as a fitness snapshot, not a promise. Marathon-specific training — long runs at marathon effort, marathon-pace blocks, fuelling practice — is what moves the prediction from hope to finish time.

Common questions

The model is reliable. Your training might not be. A 22-minute 5K runner who has never done a long run will not run the predicted 3:22 marathon. Specificity — weeks of long runs and marathon-pace work — turns prediction into performance.

Half marathon pace is closer to marathon effort than a 5K is. If you have a recent half, trust that prediction more. If your only recent race is a 5K, the number is directional — not a guarantee.

Use it as the top of your realistic range. If the model says 3:30 and you’ve trained properly, 3:30–3:45 is the honest goal window. Weather, course, and the day matter.

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