Race Time Predictor

T2 = T1 × (D2 ÷ D1)^1.06. The exponent 1.06 captures that performance degrades non-linearly with distance: double the distance and the time grows by about 2.08×, not 2×. Peter Riegel fitted it to world-record times and published it in American Scientist (1981). It tends to be optimistic beyond the marathon, so this calculator stops at standard distances and marks ultra inputs as low reliability.

Riegel's formula predicts your physiological potential, assuming you've done the right training volume for that distance, paced it correctly, and executed well. It doesn't account for insufficient long run base, poor fueling, heat, elevation, or undertrained aerobic engine. If the prediction feels unreachable, your training for that specific distance likely needs more work.

Less accurate the larger the distance gap, in either direction. A 5K → marathon prediction spreads widely because 5K is primarily VO₂max-limited while marathons are predominantly aerobic threshold and glycogen-limited. The labels follow the distance ratio: up to about 2× is reliable (10K → half marathon, half → marathon), up to about 4× is moderate, beyond that is low.

Use a result from the last 6–10 weeks where you raced close to maximum effort on a flat, road course in moderate weather. A time trial at the end of a training run, a race run conservatively, or a result from 6+ months ago will all produce less accurate predictions.