Short answer: Cycle length is counted from Day 1 of one period to Day 1 of the next period. Period length is the number of days bleeding lasts within that cycle.
What counts as Day 1?
The American College of Obstetricians and Gynecologists defines a menstrual cycle from the first day of menstrual bleeding in one cycle to the first day of menstrual bleeding in the next. In a tracker, that first day is Day 1.
In this example, the period lasted 6 days: 3 March through 8 March. The cycle lasted 28 days: from 3 March to the day before the next period began. The next cycle starts on 31 March.
Why the distinction matters
If someone says “my period is 28 days,” they may mean a 28-day cycle, not 28 days of bleeding. An app needs period start dates to build a cycle history. Bleeding duration is a separate pattern that may also change over time.
The NHS says periods commonly occur every 21 to 35 days and bleeding can last 2 to 7 days. These are broad population ranges, not a promise that every person or every cycle will match them.
A practical way to track
- Record the first day of menstrual bleeding consistently.
- Record the final bleeding day separately.
- Correct mistaken dates rather than leaving them in the history.
- Compare several cycles before treating one change as a pattern.
When a change deserves medical attention
Tracking can help you describe what changed, but it cannot explain why. NHS guidance says changes in periods may need investigation and identifies bleeding between periods, bleeding after sex, and bleeding after menopause as reasons to be checked by a doctor. Seek qualified care for concerns rather than using an app to diagnose a cause.
Sources
- ACOG: The Menstrual Cycle — last reviewed August 2024; updated May 2025.
- NHS: Periods overview — page last reviewed 5 January 2023.