Race Predictor Calculator

The Riegel formula's exponent of 1.06 is deliberately greater than 1.0, which encodes the fact that runners can't sustain the same per-mile effort over a much longer distance — factors like glycogen depletion and accumulating fatigue mean pace has to slow as distance grows. If the exponent were exactly 1.0, predicted pace would stay flat across all distances, which doesn't match how endurance performance actually degrades over longer races.

Riegel-formula predictions grow less reliable the larger the gap between your known distance and the target distance, because the formula extrapolates a single data point across a much wider range of physiological demands — a 5K primarily tests speed and lactate threshold, while a marathon tests fat-burning efficiency and glycogen management, systems that don't scale identically for every runner. Riegel's own 1981 American Scientist analysis covered performances lasting roughly 3.5 to 230 minutes, so a 5K-to-marathon prediction sits near the wide end of that range; a prediction from a recent half marathon time is generally far more trustworthy for a marathon target than one built from a 5K.

No — the Riegel formula works purely from your known finish time and the two distances involved, with no adjustment for terrain, temperature, wind, or course difficulty. It assumes your known race and your predicted race happen under comparably favorable conditions; a hilly or hot target race will likely run slower than the formula predicts, regardless of your fitness.

The same exponent that slows predicted pace for longer target distances speeds it up for shorter ones — the formula is symmetric in that sense. If your known result is a half marathon, the formula predicts a faster per-mile pace for a 5K than your half marathon pace, since shorter races place less demand on sustained aerobic endurance and more on raw speed, which the formula's exponent approximates.