How running pace is calculated
Pace is the time it takes you to run one kilometre (or one mile): divide the total time by the distance. 10 km in 55 minutes → 55 ÷ 10 = 5.5 minutes, i.e. 5:30 per km (5 minutes 30 seconds, not 5.30). Speed in km/h is the inverse: 60 ÷ 5.5 = 10.9 km/h. The first tab also shows the pace per mile, the time for a 400-metre track lap and the kilometre splits.
From km/h to min/km (treadmill)
Treadmills show speed in km/h, while running watches and apps show pace in min/km. To convert, divide 60 by the speed: at 12 km/h the pace is 60 ÷ 12 = 5 minutes per km; at 10.5 km/h it is 5.71 minutes, i.e. 5:43 per km. The third tab has a table from 6 to 20 km/h with 10K and half-marathon times.
Predicted times with the Riegel formula
From a race result the calculator estimates your times over other distances with Pete Riegel's formula: T2 = T1 × (D2 ÷ D1)1.06. The 1.06 exponent reflects the fact that pace drops a little as the distance grows. Example: a 50-minute 10K runner can aim for a half marathon in about 1:50:19 and a marathon in about 3:50:01. It is an estimate: it works well if you are trained for the longer distance, and it tends to be optimistic for the marathon without long runs.
Race distances
5K and 10K are the most common road race distances; the half marathon is 21.0975 km and the marathon 42.195 km. A mile is 1.609 km, which is why pace per mile is about 1.6 times pace per kilometre.
Frequently asked questions
What is 5:30 per km in km/h?
10.91 km/h: 5:30 is 5.5 minutes and 60 ÷ 5.5 = 10.91.
What pace do I need for a 4-hour marathon?
5:41 per km (about 9:09 per mile): 240 minutes divided by 42.195 km is 5.69 minutes, i.e. 5 minutes 41 seconds.
What pace do I need to run 10 km in 50 minutes?
5:00 per km, i.e. 12 km/h.
Does the calculator account for hills?
No: it works out the average pace as if on a flat course. Hills, wind and heat slow you down; for predictions use a recent race on a similar course.