Race Time Predictor
One honest race result predicts the others. Enter a recent finish time and get Riegel-formula predictions for every standard distance up to the marathon.
How to use this calculator
- Pick your most recent honest, all-out race — 5K and 10K predict best.
- Enter the distance and your finish time.
- Read your predicted times, and the caveat before you sign up for that marathon.
How Riegel's formula works
Peter Riegel's 1981 endurance model, published in American Scientist, fits how finish times scale with distance across sports from running to rowing:
T₂ = T₁ × (D₂ ÷ D₁)^1.06The exponent 1.06 captures fatigue: double the distance and you slow by roughly 6%. It fits trained endurance runners well between about 5K and the half marathon.
Where it breaks
- Marathon predictions run optimistic for low-mileage runners. Riegel assumes the endurance base to use your speed for 42 km. If you train under ~50 km/week, add 5–10 minutes to the prediction.
- Predictions from races under 3 km overestimate longer races — short efforts lean on speed, not endurance.
- Trail, heat and hills void the warranty — the model assumes flat road conditions.
Frequently asked questions
How accurate are the predictions?
For trained runners between 5K and half marathon, usually within a few percent. Marathon is the least reliable — the formula does not know your weekly mileage or long-run history.
My marathon was slower than predicted. Why?
Almost always endurance, not talent: the prediction assumes sufficient long runs and total mileage. Follow a proper marathon block, or treat the number as a stretch goal.
Should I predict from my fastest or my most recent race?
The most recent all-out effort on a flat course. A two-year-old PB predicts your old self, not today's.
Can I use a treadmill time?
Roughly, if the treadmill is calibrated and at 1% incline. Belt calibration drift is common — treat treadmill inputs as approximate.