Race Time Predictor

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, or a marathon from a half-marathon). 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 a 8.4× distance multiplier, and the formula's accuracy 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. When you're injured, ill, or under-trained for the target distance.

The exponent matters: 1.06 is the standard value validated against recreational runners. 1.05 is for elite runners with exceptional endurance (Kipchoge-level). 1.07 is for novice runners who slow disproportionately with distance. If your actual time at a predicted distance was slower than the formula said, your personal exponent is higher than 1.06.

What to do instead: Baseline tracks your training load (CTL/ATL/TSB) alongside your race performances. Over time, it learns your personal fatigue-resistance curve and can give you a distance-adjusted prediction that accounts for your current fitness, not just a one-size-fits-all exponent.