Race Time Predictor
Enter a recent race result and we'll predict your time at every other distance using the proven Peter Riegel formula.
Predictions assume similar conditions and appropriate training for the longer distance. The further you extrapolate, the more your endurance base matters — a 5K-to-marathon jump is optimistic if you haven't done the long runs.
The Peter Riegel formula
Published by engineer and marathoner Peter Riegel in 1981, this is the most widely used endurance prediction model in the world. It states:
T₂ = T₁ × (D₂ / D₁)1.06
where T₁ is your known time over distance D₁, and T₂ is your predicted time over distance D₂.
The exponent 1.06 captures the reality that you can't hold your 5K pace for a marathon — speed fades as distance grows. It's remarkably accurate for distances from 5K to the marathon when you have the endurance base to back it up.
How to use your prediction
- Set a goal pace. Drop your predicted time into the Pace Calculator for a printable split band.
- Be honest about training. A marathon predicted from a 5K only holds if you've built the long runs. Predicting a half from a 10K is far more reliable.
- Re-test every few weeks. As fitness improves, your predictions improve with it.
FAQ
How accurate is the Riegel race predictor?
Within a few percent for well-trained runners predicting across nearby distances (5K↔10K, 10K↔half). Accuracy drops when you extrapolate a long way — e.g. predicting a marathon from a single 5K — because the marathon depends heavily on endurance the formula can't see.
Which recent race should I enter?
Use your most recent all-out race closest to your target distance. A recent 10K or half is the best predictor of marathon performance; a 5K works but tends to be optimistic for the full.
Why is my predicted marathon slower than 8× my 5K?
Because the exponent is 1.06, not 1.0 — pace naturally slows as distance increases. That's the whole point of the model, and it's why even-pacing the marathon at "5K math" pace leads to hitting the wall.