Predict your time at a new distance from a race you've already run.
Estimate only, not a guarantee.
Peter Riegel, an engineer and runner, published a simple relationship in a 1977 Runner's World article after studying how race performances scale with distance: T2 = T1 × (D2 ÷ D1)1.06. T1 and D1 are a known time and distance, T2 and D2 are the distance you're predicting.
Take a 25:00 5K as the known result and predict a half marathon: the ratio of distances (13.1094 ÷ 3.10686) is about 4.22, raised to the 1.06 power gives roughly 4.62, and 25 minutes times 4.62 lands close to 1 hour 55 minutes. That's the same calculation the tool above runs on your own numbers.
| Target distance | Predicted time | Predicted pace |
|---|---|---|
| 5K | 25:00 | 8:03 /mi |
| 10K | 52:07 | 8:23 /mi |
| Half marathon | 1:55:00 | 8:46 /mi |
| Marathon | 3:59:47 | 9:09 /mi |
Riegel's formula (T2 = T1 × (D2 ÷ D1)^1.06), applied to a single reference 5K time; your own known time and distance above will produce different numbers.
The exponent is what separates this from a naive scaling guess. A straight ratio, multiplying the 25:00 5K time by how many times longer the marathon distance is, would land around 3:30:59, well under the Riegel estimate above. The half hour Riegel adds back accounts for the fact that holding any given pace gets harder as the distance stretches out, not just proportionally longer.
Because endurance doesn't scale linearly. Running twice as far generally takes more than twice as long, since fatigue compounds. The 1.06 exponent, from Pete Riegel's 1977 analysis of race results, captures that curve better than a straight ratio.
Yes. Riegel's formula holds up best when the two distances aren't too far apart and you've trained specifically for the longer one. Stretching a 5K time all the way to a marathon prediction assumes endurance you may not have built yet.
The prediction will be off in the same direction. A known time from a hot, hilly race will predict a slower time at the new distance than your true fitness supports, so use a race run in fair conditions if you have one.
Yes, the formula works both directions. Predicting down, from a half marathon to a 5K for example, tends to run a bit fast since it doesn't account for the extra speed training a 5K effort actually requires.