How to use this calculator
Pick what you want to calculate:
- Pace: enter a distance and your time.
- Finish time: enter a distance and the pace you plan to run.
- Distance: enter a time and a pace.
Choose miles or kilometers, then pick a race (1 mile, 5K, 10K, half marathon or marathon) or enter a custom distance. You’ll see your pace per mile and per kilometer, your speed in mph and km/h, your halfway split, your time for a 400 m track lap, a table of even-pace splits for every mile or kilometer, and predicted times for other race distances.
The formulas
Pace, time and distance are three sides of one equation:
The calculator converts everything to seconds and meters, using 1 mile = 1,609.344 m. A half marathon is 21,097.5 m (13.11 miles) and a marathon is 42,195 m (26.22 miles).
To predict a time at another distance, it uses the endurance formula Peter Riegel published in American Scientist in 1981, after fitting a simple power law to world-record performances:
The exponent of 1.06 is slightly above 1, which builds in the fact that you slow down a little as races get longer. Doubling the distance multiplies your time by about 2.08, not 2.
Worked example
You ran a half marathon in 2:00:00 and want to know your pace:
- Time: 2 hours = 7,200 seconds
- Pace: 7,200 ÷ 13.11 miles = 549 seconds = 9:09 per mile (5:41 per kilometer)
- Speed: 13.11 miles ÷ 2 hours = 6.6 mph (10.5 km/h)
- Marathon prediction: 2:00:00 × (42,195 ÷ 21,097.5)^1.06 = 4:10:12
The same formula predicts 26:05 for a 5K and 54:23 for a 10K. Going the other way, a 25:00 5K predicts 52:07 for a 10K, 1:55:00 for a half marathon and 3:59:47 for a marathon.
Pace chart for common race times
Average pace needed for popular finish times, rounded to the nearest second:
| Race | Finish time | Pace per mile | Pace per km |
|---|---|---|---|
| 5K | 20:00 | 6:26 | 4:00 |
| 5K | 25:00 | 8:03 | 5:00 |
| 5K | 30:00 | 9:39 | 6:00 |
| 10K | 45:00 | 7:15 | 4:30 |
| 10K | 55:00 | 8:51 | 5:30 |
| 10K | 1:00:00 | 9:39 | 6:00 |
| Half marathon | 1:30:00 | 6:52 | 4:16 |
| Half marathon | 1:45:00 | 8:01 | 4:59 |
| Half marathon | 2:00:00 | 9:09 | 5:41 |
| Half marathon | 2:30:00 | 11:27 | 7:07 |
| Marathon | 3:00:00 | 6:52 | 4:16 |
| Marathon | 3:30:00 | 8:01 | 4:59 |
| Marathon | 4:00:00 | 9:09 | 5:41 |
| Marathon | 5:00:00 | 11:27 | 7:07 |
A 4-hour marathon and a 2-hour half marathon need exactly the same pace, which is why running the half at that pace comfortably is a common sign you’re ready to try for 4 hours.
How reliable are race predictions?
Riegel’s formula is a good starting point, not a promise. It assumes you’re equally well trained for both distances. A runner with plenty of speed but little endurance will find the longer predictions hard to hit.
A 2016 study of recreational runners by Vickers and Vertosick found the formula worked well for predicting races up to the half marathon but substantially underestimated marathon times, producing predictions at least 10 minutes too fast for half of the runners. If you’re planning a marathon from a shorter race:
- Use your longest recent race as the input. A half marathon predicts a marathon better than a 5K does.
- Add a margin if your weekly mileage is modest or this is your first marathon.
- Check your long runs. If your easy long-run pace is close to your predicted race pace, the prediction is probably too ambitious.
Using your splits
The splits table shows where you should be at every mile or kilometer if you run an even pace. Many runners print it or write a few key splits on their wrist. Two practical tips:
- Hold back early. Adrenaline makes the first mile feel easy. Losing 10 seconds at the start is easy to recover; going out 30 seconds too fast often costs minutes later.
- Expect your watch to read long. GPS watches and the extra distance from weaving around other runners usually make your recorded distance a little longer than the certified course, so your watch pace will look slightly faster than your official average.
If you’re training regularly, your energy needs go up. The TDEE calculator estimates the calories you burn on a typical day, and the one-rep max calculator helps plan the strength work that supports your running.
Assumptions and limitations
- Pace and speed are averages over the whole distance.
- Splits assume perfectly even pacing on a flat course; hills, heat, wind and altitude all change real splits.
- Race predictions assume similar training, conditions and effort across distances, and they’re less reliable the further the predicted race is from the one you entered.
Sources
- Riegel PS. Athletic records and human endurance. American Scientist. 1981;69(3):285–290.
- Vickers AJ, Vertosick EA. An empirical study of race times in recreational endurance runners. BMC Sports Sci Med Rehabil. 2016;8:26.
These figures are estimates for planning your training and races. If you have a heart condition or other health concern, check with your doctor before starting a new running program.
Frequently asked questions
How do I calculate my running pace?
Divide your time by your distance. A 2-hour half marathon is 7,200 seconds over 13.11 miles, or about 549 seconds per mile, which is 9:09 per mile (5:41 per kilometer). The calculator does the conversion and shows your speed too.
What pace do I need for a sub-2-hour half marathon?
An average of 9:09 per mile or 5:41 per kilometer gets you to the finish in exactly 2:00:00. Most runners aim a few seconds faster to allow for crowded starts, water stations and the extra distance from not running the shortest line.
How accurate are race-time predictions?
The predictions use Riegel's formula, which raises the distance ratio to the power 1.06. In a 2016 study of recreational runners, it worked well for races up to the half marathon but gave marathon times at least 10 minutes too fast for half of the runners. Treat a marathon prediction from a 5K or 10K as a best case, especially if your weekly mileage is low.
What is a good 5K time?
It depends on age, sex and experience, so there's no single answer. A 25-minute 5K is an 8:03-per-mile pace and a 30-minute 5K is 9:39 per mile. Comparing yourself with your own past results is more useful than a general benchmark.
How do I convert pace per mile to pace per kilometer?
Divide the pace per mile by 1.609, since a mile is 1.609 kilometers. For example, 8:00 per mile is 480 seconds ÷ 1.609 ≈ 298 seconds, or about 4:58 per kilometer. Choose either unit in the calculator and it shows both.
What is a negative split?
Running the second half of a race faster than the first. Starting too fast tends to cost more time late in a race than it saves early, so an even or slightly negative split is a common goal for longer races. The splits table shows even-pace targets you can adjust from.
Updated November 2026. Results are estimates for educational purposes and depend on the inputs you provide.