Running Time Predictor

What it does

The Running Time Predictor takes a recent running result and projects what you can run at a different distance, using the common Riegel-style fatigue exponent. Useful for setting goal times when you only have one or two recent races to extrapolate from.

URL: /calculators/ then open the Running time entry. Direct path: /static/calculators/running_time_estimator.html.

Inputs the form asks for

From a known race result

Known distance (e.g. 5K, 10K, half marathon, marathon, or custom).
Time (HH:MM:SS) for that distance.

From a pace

Alternative: enter pace directly.

Pace (MM:SS min/km) or
Pace (MM:SS min/mile).

Race prediction

Distance: the target race distance.
Finish time (output): what the predictor returns.

Outputs

Predicted finish time at the target distance, in HH:MM:SS.

How the prediction works

The calculator uses a Riegel-style exponent (T2 equals T1 times (D2 divided by D1) raised to about 1.06) to scale a known time to a different distance. The exponent reflects the fatigue penalty for going longer.

The prediction is most accurate when the test and target distances are within a factor of 4 (e.g. 5K to half marathon, not 5K to ultra). Over very long extrapolations the model overestimates because metabolic limits change shape.

When to use it

Setting a goal time for a new distance from a recent shorter race.
Sanity-checking what target pace a race time implies.
Comparing two athletes' results across different race distances.

When not to use it

For ultras: physiology shifts (fueling, pacing patterns) that the model does not capture.
For very different surfaces (track vs trail).
For first-attempt distances without recent training at race pace.