🏃 Race Time Predictor
Predict your finish time at any distance
How Race Prediction Works
The Riegel formula (published in 1977 and still the standard) predicts race times using the relationship: T2 = T1 × (D2/D1)^1.06, where T1 is your known time, D1 is the known distance, D2 is the target distance, and 1.06 is the fatigue factor. The exponent accounts for the fact that you cannot maintain your 5K pace for a marathon — you slow down predictably as distance increases.
The formula is most accurate when predicting within 2-3x of your known distance. A 10K time predicts a half marathon well. A 5K time predicting a marathon is less reliable because it does not account for fueling strategy, heat management, and the mental demands of running for 3-5 hours. For best results, use your most recent race result at the closest distance to your target.
How accurate is this predictor?
Within 2-3% for most trained runners predicting similar distances (5K→10K, 10K→half marathon). Accuracy decreases for large jumps (5K→marathon) and for undertrained runners whose endurance has not caught up to their speed. If you have never run more than 10K, your predicted marathon time will be optimistic because it assumes equivalent endurance training at all distances.