Race Time Predictor

T2 = T1 × (D2 ÷ D1)^1.06. The exponent 1.06 captures the physiological reality that performance degrades non-linearly with distance — you slow down proportionally more as distance doubles. Published by Peter Riegel in Runner's World (1977) and validated across thousands of athletes. The exponent is slightly higher for ultra distances.

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. A 5K → marathon prediction has high variance (±5–15 min) because 5K is primarily VO₂max-limited while marathons are predominantly aerobic threshold and glycogen-limited. The most reliable predictions use a similar distance — e.g., 10K → half marathon, or half marathon → marathon.

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.