Estimate your finish time using the Riegel formula. Enter a recent race result to predict another distance.
The Riegel formula (T₂ = T₁ × (D₂/D₁)^1.06) is a physiological extrapolation - it assumes your endurance decays predictably with distance. Pete Riegel published it in 1977 based on world-record performances, and it's been validated across thousands of amateur runners since.
When it's accurate: For distances within a factor of 2–3 of your known time (e.g., predicting a half-marathon from a 10K). When you've been training specifically for the target distance. When the known race was recent (within 8 weeks) and on comparable terrain.
When it's wrong: Predicting a marathon from a 5K - that's an 8.4× distance multiplier, and the formula breaks down at extremes. For runners with extreme profiles (sprinters trying to run marathons, or ultra-runners predicting 5K times). Hot-weather or hilly race predictions from a flat-cool known race.
The exponent matters: 1.06 is standard for recreational runners. 1.05 is for elite runners with exceptional endurance. 1.07 is for novice runners who slow disproportionately with distance. If your actual times come out slower than predicted, your personal exponent is higher than 1.06.
Baseline tracks your training load alongside your race performances. Over time, it learns your personal fatigue-resistance curve and gives you distance-adjusted predictions that account for your current fitness.