How the race equivalency calculator works
How Riegel works
Pete Riegel's 1977 model: T₂ = T₁ × (D₂/D₁)^1.06 — performance decays sub-linearly with distance because endurance-trained humans are remarkably consistent per-mile as distances grow. The 1.06 exponent fits recreational-to-elite runners broadly; it is the engine behind most prediction features on training platforms.
The training assumption
The model predicts what equal-TRAINING performances would be — a 50-minute 10K corresponds to a ~3:47 marathon only with marathon training underneath. Without the long-run volume, predictions are ceilings, not schedules. The practical use: set marathon goals from half-marathon races within 6–8 weeks, with the long-run log to back them.
Where predictions miss
Very fast-to-very-slow extrapolations (a 5K predicting a marathon carries the most error), hot days, hilly courses, and under-fueled long races. Treat predictions as ranges: the model gives the median; your training, gut, and race-day conditions place you within it.