Race Time Predictor
Predict your race times across all distances from a single reference performance
Predicted times
Predictions based on the Riegel formula
How the predictor works
Pick a reference race
Select the distance and enter the time from a recent race. Prefer an official competition or a timed time trial for best accuracy.
Adjust the exponent
The exponent 1.06 suits most runners. Go up to 1.08 if you are more speed-oriented, or down to 1.04 if you are a pure endurance runner.
Read the predictions
The displayed times are realistic targets. They assume full recovery and normal race conditions across all standard distances.
The Riegel formula
Peter Riegel's formula (1977) predicts the time T₂ for a new distance D₂ from a known time T₁ at distance D₁: T₂ = T₁ × (D₂/D₁)^n. The exponent n (typically 1.06) captures how performance degrades as distance increases.
Limitations: the formula assumes constant fitness and does not account for training specificity, accumulated fatigue, elevation change or weather. Predictions are less reliable for very short distances (< 400m) or ultra-distances. Practical exponent guide: 1.04 for pure endurance specialists, 1.06 for most trained runners, 1.08–1.10 for speed-oriented profiles.
Frequently asked questions
Enter your most recent race time for a reference distance (e.g. half marathon or 10K in competition), choose exponent 1.06 and read the predicted time for the marathon. This method typically gives accuracy within ±5% for a well-trained runner.
The Riegel formula is an empirical equation published by Peter Riegel in 1977 in Runner's World. It models the relationship between distance and performance: T₂ = T₁ × (D₂/D₁)^1.06. The exponent 1.06 was determined by statistical analysis of thousands of race results.
Accuracy is generally ±5 to 10% for nearby distances (e.g. 5K → 10K). It degrades for very different distances (e.g. 400m → marathon). Actual performance also depends on specific training, weather conditions, and race strategy.
The standard exponent 1.06 suits most runners. If you are stronger at short distances (speed-endurance), use 1.08. If you excel at long distances (pure endurance), 1.04 will be more accurate.
Yes, but with caution. Indoor times are generally slightly faster for short distances due to tight bends (for 1500m and below). For distances above 3000m, the difference is negligible.