How to use it

Pick miles or kilometers. The preset menu fills the distance box with common road distances, from the 5K to the marathon, so you do not have to remember that a half marathon is 21.0975 kilometers.
Enter any two of the three values: distance, finish time (hours are optional for shorter runs), or pace in minutes and seconds per mile or per kilometer.
Press the button for the value you want to solve, and read the result below the form. Reset clears everything.

The math

The three values are tied together by simple identities:

pace = finish time ÷ distance
finish time = distance × pace
distance = finish time ÷ pace

The calculator works internally in seconds and kilometers, then converts for display. The international mile is defined as exactly 1.609344 kilometers, which is the conversion used here. Displayed paces and times round to the nearest second; a calculated distance shows two decimal places.

Worked example: a 10K covered in exactly 1:00:00 is 3,600 seconds over 10 kilometers:

3,600 ÷ 10 = 360 seconds per kilometer, or 6:00 per kilometer — about 10.0 km/h (6.2 mph)
The same run is 9:39 per mile, because 360 × 1.609344 ≈ 579 seconds
Cross-unit check: 8:00 per mile is about 4:58 per kilometer, so 5 km at that pace takes roughly 24:51

What the number means

The result is an average: total time divided by total distance. It does not mean every kilometer was run at that pace, and it says nothing about how the run felt. Hills, wind, surface, heat, crowding, and fatigue all change what a given pace costs on a given day — see why heat slows you down for the clearest example.

Limitations and accuracy

Results are only as good as the inputs: GPS drift, unmeasured courses, and manual timing all introduce error.
Short distances amplify timing and GPS error, so do not over-read the decimals on a 400-meter repeat.
An average pace from one course and one day does not predict a result on a different course or day.
This tool does not estimate fitness, prescribe training intensities, or judge whether a pace is medically appropriate for you.
Be consistent about what counts toward elapsed time — pauses and stoplights included or excluded — when comparing runs.

If chest pain, faintness, severe shortness of breath, or other worrying symptoms appear during a run, stop and seek appropriate medical care rather than trusting the math. This tool is general educational information, not individualized advice — see our medical disclaimer.

Sources

NIST Guide to the SI, Appendix B.8 — conversion factors, for the exact mile definition.
BIPM — The International System of Units (SI), for unit definitions.
World Athletics technical information, for standard road race distances.

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