Race predictions
Splits
The half marathon is 21.0975 km (13.11 miles). A sub-2-hour finish means holding 5:41 per km or 9:09 per mile, while 1:45 needs 4:59 per km. By Riegel’s formula a 2:00 half predicts about 4:10 for the full marathon, assuming marathon-specific training.
How to use
- Choose what to calculate: Pace, Time or Distance. The other two fields are your inputs.
- Enter the distance in km or miles, or tap 5K, 10K, Half marathon or Marathon.
- Enter the time as hours, minutes and seconds, or the pace as minutes and seconds per km or mile.
- Below the result you’ll find predicted times for other race distances and a split table for the whole run. Tap any row to copy it.
- A marathon is 42.195 km (26.22 mi) and a half marathon 21.0975 km (13.11 mi). 1 mile = 1.609344 km.
- Race predictions use Peter Riegel’s formula T2 = T1 × (D2 ÷ D1)^1.06. They assume equal training for each distance and get less reliable for very different distances.
- Splits assume an even pace. Many runners aim for even or slightly negative splits (second half faster).
- Your choice of km or miles is remembered on this device.
Pace, time and distance
Pace = time ÷ distance. Running 10 km in 50 minutes is 5:00 per km; the same pace per mile is 8:03, because a mile is 1.609344 km. Speed is the inverse: 5:00 per km equals 12 km/h or 7.46 mph.
Splits assume an even pace. Many runners aim for even or slightly negative splits, running the second half a little faster, which usually beats starting too fast.
Race predictions
Predictions use Peter Riegel’s formula, T2 = T1 × (D2 ÷ D1)^1.06. A 25:00 5K predicts about 52:07 for 10K and a 2:00:00 half marathon about 4:10:12 for the marathon. The formula assumes equal training for both distances and becomes less reliable the further apart they are; first-time marathoners often run slower than predicted.
Your choice of kilometers or miles is remembered on this device.
Frequently asked questions
How do I calculate my running pace?
Divide your time by the distance. 30 minutes for 5 km is 6:00 per km; for 3.1 miles it is about 9:39 per mile.
What pace do I need for a sub-4 marathon?
5:41 per km or 9:09 per mile, held for the full 42.195 km. Most runners add a few seconds of margin for water stations and crowds.
How do I convert pace per km to pace per mile?
Multiply the pace in seconds by 1.609344. 5:00 per km is 300 s × 1.609 = 483 s, or 8:03 per mile.
How accurate are race time predictions?
Riegel’s formula is a good estimate for distances that are close together. For the marathon it tends to be optimistic unless you have done long training runs.
What is a negative split?
Running the second half of a race faster than the first. It is a common pacing strategy for half marathons and marathons.