Race Time Predictor
Enter a recent race result to predict times for other distances.
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How Race Time Prediction Works
The Riegel formula (T2 = T1 × (D2/D1)^1.06) predicts race times based on a recent performance at a different distance. The exponent 1.06 accounts for the physiological cost of running longer.
These predictions assume equivalent training. A well-trained marathon runner may beat the prediction; someone who only trains for 5Ks may fall short at marathon distance.
The Riegel formula predicts finish time for a new distance from a known performance: T2 = T1 × (D2/D1)^1.06. The exponent 1.06 reflects the physiological cost of longer distances — you can't sustain the same pace for twice the distance. The formula is most accurate for distances from 1 mile to marathon; it underestimates ultra performance.
Example: you ran a 5K in 24 minutes. Predicted 10K: 24 × (10/5)^1.06 = 24 × 2^1.06 = 24 × 2.083 = 50 minutes. Predicted half marathon: 24 × (21.1/5)^1.06 = 24 × 4.22^1.06 = 24 × 4.54 ≈ 109 minutes (1:49). These are physiological predictions assuming comparable training and conditions.
The prediction assumes you're equally trained for both distances. If you're a strong 5K runner who hasn't done long runs, the marathon prediction will be optimistic — lack of long-run-specific adaptations (glycogen storage, fat oxidation, musculoskeletal durability) limits longer race performance beyond what the formula captures.
Use race predictions for goal-setting and pacing, not as certainties. Weather, elevation, course profile, and racing experience all affect actual race performance by 5–15% in typical training conditions.
See also: Pace Calculator · VO2max Estimator