Running Pace Calculator and Race Time Predictor
Enter a distance and a time to get your average pace per kilometre and per mile, your speed, and predicted times for 5K, 10K, half marathon and marathon from Riegel's formula. The prediction assumes you've trained for the longer distance. It's an estimate, not a promise.
Distance unit
Time
Advanced
Race time predictions (Riegel)
| Distance | Time | Pace |
|---|---|---|
| 5 km | 23:59 | 4:48 /km |
| 10 km | 50:00 | 5:00 /km |
| Half marathon | 1:50:19 | 5:14 /km |
| Marathon | 3:50:01* | 5:27 /km |
* Outside 3.5–230 min, the estimate is less reliable.
The prediction holds if you've trained for that distance. For the marathon, the formula is often too fast.
Before you set a goal
- Predict from a recent race run all-out on a flat course, not from a training run. Riegel's formula scales the effort you put in, so an easy run gives an easy prediction.
- Predictions from a nearby distance are the most reliable: 10K to half marathon works better than 5K to marathon. The further you stretch it, the bigger the error.
- For the marathon, the formula is often too optimistic. In a study of over 2,000 recreational runners, half of them ran at least 10 minutes slower than predicted. Without long runs and enough weekly mileage, add a margin.
- GPS watches often measure a race slightly longer than its official distance. To compare with the chart, enter the official distance (for example, 42.195 km), not what your watch shows.
How this is calculated
Pace is your time divided by the distance: 50 minutes over 10 km is 5:00 per kilometre. Per-mile pace uses the international mile of exactly 1.609344 km, and speed is distance divided by time in hours. The race predictions use the formula published by Peter Riegel in Runner's World in 1977 and expanded in American Scientist in 1981: predicted time = known time × (new distance ÷ known distance)^1.06. The exponent above 1 means your pace slows as the distance grows. Doubling the distance multiplies the time by about 2.08, not 2. Riegel fitted the formula to performances lasting about 3.5 to 230 minutes, so predictions outside that window are marked as less reliable. You can change the exponent: 1.06 is the most widely used value, and research on recreational runners has used 1.07–1.08. Times and paces are rounded to the nearest second.
Common questions
Why is my predicted marathon time faster than I actually run?
Because the formula assumes you're as well trained for the marathon as for the race you entered. Riegel built it from top performances, and a study of recreational runners found it was accurate up to the half marathon but too fast for the marathon, by at least 10 minutes for half of them. If your long runs are short, use a higher exponent (1.08–1.10) or treat the prediction as a best case.
What exponent should I use?
1.06 is the value most calculators use and the default here. Some researchers use 1.07 as an average, and some analyses of recreational marathoners found values around 1.15. If you've raced two distances recently, the exponent that links them is your personal one. A lower value suits endurance-minded runners, a higher one suits runners who are fast over short distances.
Can I use it for a 100 km ultra or a 400 m sprint?
The pace and speed are correct at any distance, since that's simple division. The prediction isn't: Riegel's formula covers efforts of roughly 3.5 minutes to just under 4 hours, so anything shorter or longer is flagged. Ultras depend on pacing, food and terrain far more than the formula can capture.
Part of Sports & Fitness Calculators