Race Time Predictor
Predict your time at another distance from a recent result, by the Riegel formula.
Equivalent times
1500 m 06:01 Mile 06:29 3000 m 12:33 5 km 21:35 10 km 45:00 15 km 1:09:09 10 miles 1:14:31 Half marathon 1:39:17 Marathon 3:27:00
Where this comes from
- Rule
- T₂ = T₁ × (D₂ ÷ D₁) to the power 1.06
- Source
Give a recent result and see what it is worth at every other distance. The formula is Pete Riegel's, published in Runner's World in 1977 and still the one most race predictors use — named here, with the range it was fitted over.
How it works
The prediction is the old time multiplied by the ratio of the distances raised to the power 1.06. The exponent is the whole formula: at 1.0 it would assume pace holds however far you go, which nobody's does. At 1.06, doubling the distance multiplies the time by about 2.08 rather than by 2.
Riegel fitted it to race and time-trial data for trained athletes and gave it a working range of roughly three and a half to two hundred and thirty minutes — the 1500 m to the marathon. Predictions falling outside that range are marked, because the arithmetic still works there and means less.
He also observed the exponent nearer 1.08 for elite runners and nearer 1.05 for older recreational ones. A single exponent for everybody is the formula's main simplification, and one more reason to read the output as an estimate.
Examples
| Case | Input | Result |
|---|---|---|
| From a 10 km | 10k in 45:00 | half 1:39:17, marathon 3:27:00 |
| From a 5 km | 5k in 20:00 | 10 km 41:41, marathon 3:11:49 |
Frequently asked questions
Why is the marathon prediction always too fast?
Because the formula assumes you have trained for the distance you are predicting. It converts fitness between distances; it cannot know whether you have run far enough in training to hold that fitness for three hours. Predicting a marathon from a 5 km is where that assumption does the most damage.
What does the exponent 1.06 mean?
How fast pace decays with distance. It is fitted from data, not derived: Riegel found 1.06 across running, swimming and cycling for trained athletes. A lower exponent would mean pace holds better over distance, a higher one that it falls away faster.
Is there a better formula?
There are others — Cameron's, and Daniels' VDOT tables — and they disagree with Riegel and with each other, mostly at the extremes. None is authoritative. What matters more than the choice is knowing that all of them are estimates fitted to other people's races.
What does "outside the fitted range" mean?
That the prediction is shorter than about three and a half minutes or longer than about three hours fifty — the window Riegel worked in. The arithmetic still produces a number there. It just has nothing behind it.
Good to know
- An estimate of what a result is worth elsewhere, not a target and not a promise. It knows nothing about your training, the course, the weather or the day.