Two runners finish five kilometres in 25 minutes. One holds the same pace throughout. The other starts quickly and slows each kilometre. The finishing time hides the difference; running splits reveal it.

Use lap times to inspect individual sections and cumulative splits to see elapsed time at each marker. Labels vary between watches and apps, so check which number you are recording before comparing sessions.

Read one example two ways

Here is an illustrative run over five equal one-kilometre sections:

Kilometre	Time for this kilometre	Elapsed time
1	4:40	4:40
2	4:50	9:30
3	5:00	14:30
4	5:10	19:40
5	5:20	25:00

The average is 5:00 per kilometre, but no average tells the whole story. The last kilometre took forty seconds longer than the first. That is a reason to investigate pacing, terrain and conditions, not proof of one particular mistake.

A hill, a road crossing or a headwind could explain a slower section. Compare similar segments before deciding that your fitness or effort changed.

Record the recovery too

Suppose your repeated efforts all take ninety seconds, but the rest between them grows from one minute to two. The effort times look consistent while the session is changing underneath them.

Write down recovery duration alongside each effort when it matters to your plan. You can then distinguish holding a pace under the same conditions from holding it after progressively longer breaks.

A useful note might be: six timed efforts, recovery stayed at ninety seconds, last two felt harder. There is no need to turn every run into a spreadsheet. Record enough to answer the question you care about next time.

Choose what is fixed and what is measured

For a distance-based repetition, the distance is fixed and time is the result. A stopwatch or running watch with lap recording can measure it.

For a time-based repetition, the duration is fixed and distance may vary. An interval timer can tell you when to switch from an effort to recovery. This website does not record laps, measure distance or calculate your pace.

That means a ninety-second interval is not automatically a 400-metre repetition. At 6:00 per kilometre, ninety seconds covers 250 metres if pace is constant. At 4:00 per kilometre, it covers 375 metres. Use your own pace when translating between time and distance.

Make comparisons fair enough to help

Start and end each recorded section at consistent points. Note whether you included standing rest, walking recovery or a warm-up in the total. Keep units consistent too: a mile split is not comparable to a kilometre split without conversion.

For a timed run/walk session, the 1:1 running timer provides equal periods. For another ratio, edit its seconds fields. The running interval guide explains how to choose and enter a schedule without assuming a particular pace.

If you need audio while a phone is locked in your pocket, test it carefully. Browser sound can be delayed or suppressed in the background. A sports watch or dedicated running app may fit that situation better.

A useful training note should help you make the next decision. It might show a steadier pace, a recovery that changed, or simply a route that is too uneven for clean comparisons. The numbers are clues, not a verdict.

Try it: use a fixed interval when time is the part you want to control.