Race Time Predictor
Predict your finish times at other distances from one recent race using the Riegel formula — see equivalent 5K, 10K, half marathon, and marathon times.
| 5K time | 1 mile | 3K | 5K | 10K | 15K | 10 mile | Half marathon | Marathon | 50K (ultra) |
|---|---|---|---|---|---|---|---|---|---|
| 18:00 | 5:25 | 10:28 | 18:00 | 37:32 | 57:41 | 1:02:09 | 1:22:48 | 2:52:38 | 3:26:40 |
| 20:00 | 6:01 | 11:38 | 20:00 | 41:42 | 1:04:05 | 1:09:03 | 1:32:00 | 3:11:49 | 3:49:38 |
| 22:30 | 6:46 | 13:06 | 22:30 | 46:55 | 1:12:06 | 1:17:41 | 1:43:30 | 3:35:48 | 4:18:20 |
| 25:00 | 7:31 | 14:33 | 25:00 | 52:07 | 1:20:07 | 1:26:19 | 1:55:00 | 3:59:47 | 4:47:02 |
| 30:00 | 9:01 | 17:27 | 30:00 | 1:02:33 | 1:36:08 | 1:43:35 | 2:18:00 | 4:47:44 | 5:44:27 |
Riegel-formula estimates. For an exact prediction from your own time, use the predictor on the home page.
Ran a recent race? This race-time predictor estimates what you could run at other distances using the Riegel formula, T₂ = T₁ × (D₂ ÷ D₁)1.06 — a well-known endurance model. The chart above reads from a range of reference times so you can find one close to yours; for an exact prediction from your own time, use the predictor on the home page.
It's a reality check for goal-setting: a 22-minute 5K, for example, points to roughly a 45-minute 10K and a 1:40 half marathon. The further the target distance is from the race you actually ran, the rougher the estimate — predicting a marathon from a 5K is optimistic because it can't see your endurance or fuelling.
Use it the smart way: predict down or across short gaps (5K → 10K, 10K → half) for the most reliable numbers, and treat long extrapolations (5K → marathon) as a ceiling you still have to train for.
Frequently asked questions
How does a race time predictor work?
It applies the Riegel formula, which scales your time by the ratio of distances raised to the power 1.06. In plain terms: longer distances are run at a slightly slower pace, and the formula estimates by how much.
How accurate is the Riegel prediction?
It's reasonably accurate when the two distances are close (within a 2–3× range) and you're trained for both. It tends to be optimistic for big jumps like 5K to marathon, where endurance and fuelling dominate.
Can it predict a marathon from a half marathon?
Yes, and the half-to-marathon prediction is one of the more useful ones — though most runners finish a little slower than the raw formula suggests unless they've done marathon-specific long runs.
What race should I base my prediction on?
Use your most recent, well-paced race at a distance not too far from your target. A fresh 10K is a great basis for half-marathon planning; a recent half is best for the marathon.