Estimate your time over another distance from a recent result with Riegel's formula. Worked examples from 5K to marathon, and the limits of the estimate.
Start from a real result
The best estimate starts from a recent race (less than two months old), run all out, on a flat course. A 10K is ideal: long enough to reflect endurance, short enough to be raced often.
Riegel's formula
Published by engineer Peter Riegel in 1977, it is still the reference:
T2 = T1 × (D2 / D1)^1.06
T1 is your time over distance D1, T2 the estimated time over distance D2. The 1.06 exponent reflects the natural slowdown as distance grows.
Examples from a 10K
| 10K time | 5K | Half marathon | Marathon |
|---|---|---|---|
| 40:00 | 19:11 | 1:28:15 | 3:04:00 |
| 45:00 | 21:35 | 1:39:17 | 3:27:01 |
| 50:00 | 23:59 | 1:50:19 | 3:50:01 |
| 55:00 | 26:23 | 2:01:21 | 4:13:01 |
| 1:00:00 | 28:47 | 2:12:23 | 4:36:01 |
To work from any distance, Runometry's race time predictor applies the formula and gives the pace to hold.
The limits of the estimate
- The marathon is often overestimated: the formula assumes training suited to the distance. Without regular long runs (2 hours or more), add 5 to 10%.
- Elevation and heat are not taken into account: a hilly course or a summer race easily costs several minutes.
- Trail running does not fit this calculation: time depends mostly on elevation and terrain.
An estimate, not an individual prediction
Formulas like Riegel's describe an average observed across many runners. For a single person, the gap can be large, because many other factors come into play:
- Running economy: at the same speed, some runners use less energy thanks to their stride, their weight or their gear.
- The ability to hold a high percentage of your thresholds: two runners with the same VMA (maximal aerobic speed) will not hold the same share of it, or of their second threshold, over a half or a full marathon. Endurance can be trained, and it makes the difference over long distances.
- Mindset: handling the discomfort of the final kilometres matters as much as fitness.
- Race-day condition: sleep over the previous nights, fatigue, a minor illness, stress.
- Nutrition: glycogen stores before the race and fuelling during it, decisive beyond 90 minutes.
- Course profile: changes of pace, turns, climbs and descents.
- Weather: heat, humidity and wind can cost several minutes.
- On trails: terrain technicality (rocks, roots, mud) and elevation make these formulas unusable.
So treat the estimate as a ballpark figure, to compare with how you feel in training.
Turn the estimate into a goal
Use the estimate as the upper end of your range, then set a slightly more cautious goal for your first race over the distance. All that is left is choosing the date: upcoming marathons and half marathons.