How this calculator works
A half marathon is exactly 21.0975 kilometres, or 21,097.5 metres. Pace is the time taken per kilometre, so the finish time in seconds divided by 21.0975 gives seconds per kilometre. Multiplying a target pace by 21.0975 gives the finish time. The split table uses 5 km checkpoints because most races mark them, with the final row covering the remaining 1.0975 km. Even pacing is the baseline: the 5 km blocks take the same time until the short final segment. A slightly faster second half is common, but the even-split table gives a neutral target to adjust from.
Formula
Pace (seconds per km) = finish time in seconds x 1000 / 21,097.5. Finish time = pace x 21.0975. Pace per mile = pace per kilometre x 1.609344. Average speed in metres per second = 21,097.5 / finish time in seconds.
Worked example
A 1:45:00 half marathon is 6,300 seconds. Dividing by 21.0975 km gives 298.61 seconds per kilometre, or 4:59. The mile pace is 298.61 x 1.609344 = 480.57 seconds, or 8:01. Each 5 km split then falls at 24:53, 49:46, 1:14:39 and 1:39:32. Holding exactly 5:00 per kilometre gives a finish time of 300 x 21.0975 = 6,329.25 seconds, or 1:45:29.
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Frequently asked questions
How should I pace a half marathon?
Even or slightly negative splits are the usual target. Starting faster than goal pace is common but often costs time late in the race. Use the 5 km splits as checkpoints and keep the first one controlled.
Does this allow for hills or weather?
No. The calculation is a flat, still-air pace. Hills, heat, wind and crowded aid stations all add time, so adjust the target on race day rather than forcing the table exactly.
What is the difference between half marathon and marathon pace?
A marathon is twice the distance, and most runners hold a slower pace there. A recent half marathon time is a useful input to a race predictor, but it does not set marathon pace on its own.
Why does the last split not cover a full 5 km?
The half marathon is 21.0975 km, which is four full 5 km blocks plus 1.0975 km. The final table row shows the elapsed time at the finish, so its distance and split length are shorter than the earlier rows.