McMillan Race Time Equivalents

Predict equivalent race times for other distances based on a recent race, using the McMillan exponent (similar to the Riegel formula).

Inputs

Result

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How to use this calculator

  • Enter a recent race distance and time.
  • Read the predicted equivalents for 1 mile, 5K, 10K, half, and full marathon.
  • Use as a target — your actual race may differ ±2-5%.

About this calculator

McMillan's race-equivalent table uses an exponent very close to Riegel's 1.06 to predict equivalent race times across distances. The model assumes "all-else-equal" performance — same fitness, same conditions, same training. Real-world results vary because longer races stress endurance, hydration, and pacing in ways the formula cannot model.

Frequently asked

How accurate are these predictions?+
For distances within 2x of your base race, typically within 2-3%. For going from 5K to a marathon (8x), real times are usually slower than predicted unless you specifically train for the marathon.
How is McMillan different from Riegel?+
They use almost identical exponents (~1.06). McMillan publishes a more elaborate set of training paces alongside his predictor; Riegel is the underlying race-time formula.
Why is my marathon prediction so much faster than my actual time?+
The formula assumes you are equally trained for all distances. Marathons require fueling, pacing, and durability that aren't in the equation. Subtract 1-2 minutes per mile if you haven't done long runs.
Can I use a half-marathon time as my base?+
Yes — the formula works in both directions. Predicting "down" (e.g. 5K from a half) is generally more accurate than predicting "up".
Should I use this for ultra distances?+
No. The 1.06 exponent under-predicts time loss above the marathon distance. Ultra-specific calculators (e.g. UltraSignup) use different models.

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