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How the Predictor Works

The predictor uses Jack Daniels' VDOT formulas — the same math used by most modern training plans. Your race time gives a VDOT (a fitness number); the VDOT then predicts your time at any other distance.

For the math behind VDOT, see Jack Daniels' VDOT explained. For training paces from your VDOT (easy, marathon, threshold, intervals, repetitions), use the VDOT calculator.

How Accurate Is It?

Within 2-3% for most runners on similar conditions. The predictions are most reliable for distances close to your input race. Some honest caveats:

  • 5K → marathon: Tends to overpredict. The 5K tests VO2max; the marathon tests endurance. Most runners' actual marathon time is 5-15% slower than a pure VDOT prediction from their 5K.
  • Marathon → 5K: Tends to underpredict slightly. Marathon training builds endurance at the cost of top-end speed.
  • 10K → half marathon: Highly accurate. The two distances share most physiology.
  • Half marathon → marathon: Reasonably accurate if you've done long-run training. Expect to run 10-20 minutes slower than a 2× half marathon if you haven't.

The Honest Limits

The predictor doesn't know:

  • Course profile. Hilly courses are 2-5% slower than flat predictions.
  • Weather. Heat slows you down — see heat pace adjustment for the math.
  • Your training history. A runner who's done four marathons will hit predictions; a first-timer often won't.
  • Your fueling and pacing strategy. A poorly-paced or under-fueled marathon undershoots the prediction by 10-30%.

Use the predictor as a fitness ceiling — what you're capable of in ideal conditions. Adjust downward for everything else.

What to Do With the Numbers

Three useful applications:

  1. Set realistic race goals. If you've run a 22:00 5K, your VDOT predicts a 3:36 marathon. If you're targeting sub-3:30, you have work to do — at least 12-16 weeks of training to lift your fitness ceiling.
  2. Check goal feasibility. If you're chasing sub-3 and your recent 10K predicts a 3:15 marathon, sub-3 is unrealistic right now. Either reset the goal or build to a 38-minute 10K first.
  3. Pick a tune-up race. Race a 10K six weeks before your goal half marathon. Plug the time into the predictor. If it predicts your goal half time, you're on track. If not, adjust race expectations honestly.