Predict your finish time for a 5K, 10K, half marathon or marathon from a recent race result, with the pace you’d need per km and per mile.
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How the prediction works
The calculator uses Pete Riegel’s formula, published in 1977 and still widely used: T2 = T1 × (D2 ÷ D1)1.06. The 1.06 exponent reflects how runners slow down as distances get longer.
Predictions from a 5K time
| 5K time | 10K | Half marathon | Marathon |
|---|---|---|---|
| 20:00 | 41:42 | 1:32:00 | 3:11:49 |
| 25:00 | 52:07 | 1:55:00 | 3:59:47 |
| 30:00 | 1:02:33 | 2:18:00 | 4:47:44 |
Example: 50-minute 10K
A 50:00 10K predicts a marathon of about 3:50:01, which is 5:27 per km.
Getting a realistic prediction
- Use a recent, all-out race, not a training run.
- Predict from the nearest distance you’ve raced: a half marathon predicts a marathon better than a 5K.
- If you rarely run long, raise the fatigue factor to 1.08 or more for longer races.
- Heat, hills and wind can add minutes.
Frequently asked questions
How accurate is the Riegel formula?
It’s accurate for well-trained runners between 5K and half marathon. Marathon predictions are often a little optimistic unless you’ve done the long runs.
What will my marathon time be if I run a 25-minute 5K?
About 3:59:47 using Riegel’s formula, if you’re trained for the distance.
What is a good half marathon time?
Many recreational runners finish between 1:50 and 2:30. Your prediction depends on your current fitness.
What pace do I need for a sub-4 marathon?
About 5:41 per km, or 9:09 per mile.
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