Support article
Cycle length is one of the simplest numbers in women’s health tools, and it becomes much more useful when you count it the same way every month.
Article body
The first day of bleeding is day one. Keep counting until the day before the next period starts. That full count is your cycle length.
Once you know the range of your cycle, the next-period estimate becomes more useful and more honest.
If your shortest and longest cycles are far apart, a range-based forecast is usually better than forcing one exact due date.
If you want the fastest way to measure the gap between two period starts, use the cycle length calculator first and move into the irregular page later if the numbers vary a lot.
Measure the cycle length from one period start to the next, then reuse it across the rest of the site tools.
FAQ
Cycle length runs from the first day of one period to the first day of the next period.
Period length only counts bleeding days. Cycle length covers the entire month between period starts.
If the number changes a lot each month, track the shortest and longest values instead of forcing one average.