Marathon Time Predictor - Race-Based Estimate

Estimate a marathon from one shorter race, derive a personal exponent from two races, or work backward from a marathon goal.

Solve for

Required: the source race performance or performances shown for the selected mode.

Source distance

Source distance custom value

Source distance custom unit

Source race time hours

Source race time minutes

Source race time seconds

Marathon goal time hours

Marathon goal time minutes

Marathon goal time seconds

Race 1 distance

Race 1 distance custom value

Race 1 distance custom unit

Race 1 time hours

Race 1 time minutes

Race 1 time seconds

Race 2 distance

Race 2 distance custom value

Race 2 distance custom unit

Race 2 time hours

Race 2 time minutes

Race 2 time seconds

Fatigue exponent

How the Marathon Time Predictor Works

This page fixes the target at 42,195 meters and uses shorter-race performance to estimate marathon time. It adds marathon pace, exponent scenarios, and even-pace checkpoints.

One-race mode uses a selected fatigue exponent. Two-race mode derives a personal exponent, while reverse mode finds the shorter-race time associated with a marathon goal.

Formula and Method

Formula: Tmarathon = Tsource x (42195 / Dsource)^k

The scenario table recalculates at k values of 1.06, 1.08, and 1.10. Two-race mode uses k = ln(T2/T1) / ln(D2/D1).

Choosing the Right Inputs

Use a recent race shorter than the marathon and representative of current fitness. Two-race inputs should be comparable all-out efforts.

How fatigue exponent changes a marathon estimate
Exponent	Projection effect	Use
1.06	Fastest scenario	Classic Riegel starting point
1.08	More fatigue	Sensitivity check
1.10	Most fatigue of these rows	Conservative sensitivity check

Understanding Your Results

The predicted finish leads, followed by pace, three exponent scenarios, and 5K through 40K checkpoints.

Scenario spread is more honest than presenting one exact-looking answer. A personal exponent is still a projection, not a race-day guarantee.

Worked Example

A 1:45 half marathon at k = 1.06 predicts about 3:38:55. A 45:00 10K predicts about 3:27:01.

Common Mistakes

Do not mix training-run and race-effort results, use a source distance at or above the marathon, or confuse even-pace checkpoints with a pacing prescription.

Assumptions and Limitations

Research in recreational runners shows a fixed Riegel formula can underestimate marathon time. The tool does not model preparation, fueling, terrain, temperature, wind, or race execution.

Sources

Peter S. Riegel. Athletic Records and Human Endurance. American Scientist, 1981.
Andrew J. Vickers and Emily A. Vertosick. An Empirical Study of Race Times in Recreational Endurance Runners. BMC Sports Science, Medicine and Rehabilitation, 2016.