Race Time Predictor
Predict your finish time for a target race distance from a recent result at another distance, using Riegel's endurance formula T₂ = T₁ × (D₂/D₁)^1.06. Enter a recent race (distance + finish time) and a target distance; get the predicted time, pace, and speed. Supports common race distances (mile, 5K, 10K, half marathon, marathon) and metric or U.S. units. The formula is an approximation that works best for distances within a few-fold of each other; elite runners and short-to-long jumps can deviate. Everything runs locally in your browser.
Riegel's formula (1977/1981): T₂ = T₁ × (D₂/D₁)^1.06, where T and D are time and distance. The exponent 1.06 models the slow-down as distance grows. It is most accurate when the two distances are within roughly a 4× range; predictions for large jumps (e.g. 1 mile → marathon) tend to be optimistic for most runners. Pace is shown per kilometre / per mile. Runs locally.