Advanced options
Riegel formula: predicted time = known time x (new distance / known distance) ^ 1.06. Reliable between neighbouring distances; marathon predictions from a 5k assume the endurance training exists. Predictions are targets for evenly-paced racing, not guarantees.
What The Race Time Predictor Works Out
- 5k, 10k, half marathon and marathon predictions: Pete Riegel's 1977 formula scales a known race time to a new distance by the ratio of distances raised to the power 1.06. A 25-minute 5k predicts about 52 minutes for 10k, 1:55 for a half marathon and just under four hours for the full, the exponent encoding the empirical truth that pace decays about 6% per doubling of distance.
- Its honest limit: the formula fits real results across abilities remarkably well between neighbouring distances, but a 5k time knows nothing about whether marathon endurance exists. Predictions sharpen when the input race is recent, honest and at a neighbouring distance; marathon predictions firm up once long runs pass 28 to 30 km, and a tune-up half six to eight weeks out recalibrates them with current fitness.
- Hours (advanced): adds whole hours to the minutes box so a half marathon or marathon time can be entered naturally.
- Fatigue exponent and training profile (advanced): the formula systematically flatters runners moving up in distance without the weekly mileage, for whom the exponent drifts from 1.06 toward 1.10 and beyond, which is minutes at the marathon. High-mileage endurance runners can hold 1.04. The sensitivity card shows the target race at both extremes.
- Target race (advanced): any common UK race distance from a mile to 20 miles, with its predicted time, pace per km and per mile and speed. The practical use of a prediction is pacing, not prophecy: even pacing or slight negative splits is how predicted times actually get run, and the classic failure is the first 10k of a marathon run at half-marathon pace on race-day adrenaline.
- Course and conditions penalty (advanced): heat, wind and hills all cost real minutes the formula cannot see. Add 2% to 5% for a warm day or an undulating course and more for a hilly trail race; the prediction is otherwise the ceiling on a fair day.
- What your goal needs (advanced): reverses the formula to show the time you would need at your recent distance for the goal to be realistic, for example the 5k that a sub-4-hour marathon implies, and how far the goal sits from the current prediction.
- Full distance table (advanced): predictions and paces for nine distances, so a spring 10k can set an autumn marathon target. Riegel's descendants power most online calculators and the age-grading tables.