Race Time Predictor

How race time prediction works

This calculator uses the Riegel formula, T2 = T1 × (D2/D1)^1.06, which models how finish time scales with distance based on the mild fatigue effect endurance athletes experience as races get longer.

Predictions are most accurate when your two distances are reasonably close together and your recent race was run at genuine race effort. Predicting a marathon time from a 5K result is a much rougher estimate than predicting a 10K time from a 5K result, since marathon performance depends heavily on fueling and long-run endurance that a 5K doesn't test.

Use the result as a training-pace guide, not a promise — specific endurance work for the target distance still matters.

Where these numbers come from

The formulas and default values in this calculator are based on:

• Riegel, P. (1977), "Athletic Records and Human Endurance," American Scientist — exponential fatigue model for race time prediction

These are planning estimates for training purposes. Check with a coach, trainer, or physician before starting a new program or maxing out.