Tinker 
How to use it
Type the distance, choose kilometres or miles, then enter your time in hours, minutes and seconds. The calculator shows pace in both units, your average speed, and how long the 5K, 10K, half marathon (21.0975 km) and marathon (42.195 km) would take if you held the same pace.
The formula
Pace is time divided by distance. The tool converts your time to seconds and your distance to kilometres (1 mile = 1.609344 km), then divides. Pace per mile is pace per kilometre multiplied by 1.609344. Speed is the reverse: distance divided by time in hours.
Worked example
A 5 km run in 25:00 is 1,500 seconds. 1,500 ÷ 5 = 300 seconds, or 5:00 per km. Multiply by 1.609344 to get 482.8 seconds, which rounds to 8:03 per mile. Speed is 5 ÷ (1,500 ÷ 3,600) = 12 km/h, about 7.46 mph. At that pace a 10K takes 50:00 and a marathon 3:30:58.
Practical tips
Race projections assume an even pace, so treat them as estimates: most runners slow over longer distances, and formulas like Riegel's add roughly 5–6% per doubling of distance. Use your projected time as an upper target and pace easy runs 60–90 seconds per km slower than race pace.
Frequently asked questions
What is a good 5K pace?
Many recreational runners finish 5K in 25–35 minutes, a pace of 5:00–7:00 per km (8:03–11:16 per mile). Any steady improvement on your own time is a good result.
How do I convert min/km to min/mile?
Multiply the pace in seconds by 1.609344. For example, 6:00/km is 360 s × 1.609344 = 579 s, or 9:39 per mile.
Why is my watch pace different?
GPS watches measure distance with some error, especially among buildings and trees, and display rolling pace. Enter the official course distance for the most accurate average.
Can I predict my marathon time from a 10K?
This tool shows the even-pace time, which is optimistic. Expect a real marathon to be slower unless you have trained specifically for the distance.