How this calculator works
Pace = duration / distance; duration = pace × distance; distance = duration / pace. Speed = distance / hours. Internal time is seconds and internal distance is kilometers. One international mile is exactly 1.609344 km.
Enter times as min:sec (25:00) or hr:min:sec (1:25:00). A bare 25 is ambiguous and rejected. Seconds and the middle component of hr:min:sec must be below 60; fractional seconds are allowed. Pace uses the selected kilometer or mile unit.
Presets set distance to 5 km, 10 km, 21.0975 km or 42.195 km and select pace calculation. Each whole kilometer or mile gets equal time; the final partial split gets time in proportion to its length. Switch display units to see the other split system.
Reading your result
This is even-pace arithmetic, not a forecast of running ability. Weather, elevation, stops, course measurement and fatigue can change an actual finish. Each displayed split and elapsed time rounds independently to whole seconds.
Illustrative worked example
5 km in 25:00 gives 5:00 per km and 12 km/h. In miles this is approximately 8:03 per mile over 3.1069 miles; the last 0.1069 mile is a partial split.
Supported inputs & limitations
Adult recreational planning with distances 0.1–200 km, durations up to 48 hours, and pace 1:00–60:00 per km. Zero, negative and out-of-scope inputs or calculated results are rejected. The selected output field is not used as an input or included in a sharing link.
Sources & methodology
These primary sources support the method and context. They do not endorse helsi or verify this implementation.