Race Time Predictor - Predict Running Times

Predict a running time across race distances, work backward from a goal, or use two performances to calculate a personal fatigue exponent.

Solve for

Required: the distances and times shown for the selected mode.

Source distance

Source distance custom value

Source distance custom unit

Source time hours

Source time minutes

Source time seconds

Target goal time hours

Target goal time minutes

Target 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

Prediction target distance

Prediction target distance custom value

Prediction target distance custom unit

Fatigue exponent

1.06 is the standard Riegel value; the supported custom range is 1.00 to 1.20.

How the Race Time Predictor Works

The predictor scales a known performance by the ratio between target and source distance. A fatigue exponent accounts for the slowing that normally appears as races get longer.

Use one-race mode for a quick projection, reverse mode for a qualifying target, or personal mode when two recent comparable efforts are available.

Formula and Method

Formula: T2 = T1 x (D2 / D1)^k

The standard exponent is 1.06. Personal mode derives k from two results with k = ln(T2 / T1) / ln(D2 / D1), then anchors the projection on the longer race.

Choosing the Right Inputs

Use performances that reflect similar fitness and race effort. Custom distances are converted to meters before the formula runs.

What changing the fatigue exponent does
Exponent	Typical use	Effect at longer distances
1.06	Standard Riegel starting value	Fastest of these example projections
1.08	More fatigue allowance	Moderately longer prediction
1.10	Still more fatigue allowance	Longest prediction

Understanding Your Results

The primary time is followed by kilometer and mile pace, speed, distance ratio, time multiplier, and the exponent used.

A higher exponent produces a slower long-distance prediction. The number is a performance model, not a promise.

Worked Example

A 20:00 5K with k = 1.06 predicts a 41:42 10K. A 45:00 10K predicts a 1:39:17 half marathon.

Common Mistakes

Do not compare unequal-effort races in personal mode, reverse the distance ratio, or reuse rounded pace as an input to another step.

Assumptions and Limitations

Riegel’s original endurance range was approximately 3.5 to 230 minutes. Recreational marathon outcomes can be slower than a fixed-exponent projection because training, fueling, weather, terrain, and durability are not inputs.

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.