Why rounded race splits can miss your finish time

Separate exact pace from displayed pace

The numbers in this example are illustrative, not a customer training record.

A 10 km target of 43:17 is 2597 seconds. Dividing by ten gives an exact average of 259.7 seconds per kilometre, or 4:19.7. A whole-second display rounds that to 4:20.

If you repeat the displayed 4:20 ten times, the sum is 43:20—three seconds longer than the original goal. Neither subtraction nor time formatting is broken. The difference comes from rounding the same fraction at every kilometre.

Round cumulative times instead

Calculate each exact cumulative checkpoint, round it, then subtract the previous displayed cumulative time. The displayed splits alternate where needed, while the final cumulative total remains the requested 43:17.

Account for a partial final split

A half marathon or marathon does not finish at an exact whole-kilometre checkpoint. A pace band must include the remaining partial distance. Treating that section as another full kilometre gives the wrong finishing total.

The same issue appears when showing mile splits for a metric distance. Keep the exact event distance during the calculation, then format the checkpoint labels. Rounding the distance too early introduces a different error from rounding the time.

Check the band before printing

Enter your target in the race-splits and pace-band tool. Read the final cumulative time and the final partial distance, not only the average pace. If you change the target or strategy, calculate again before printing.

The arithmetic describes an intended schedule. A watch’s GPS distance, the line you actually run and race conditions can differ from the assumed course distance. Keep those practical differences separate from the rounding check.