How to Calculate Menstrual Cycle Length
Menstrual cycle length is the number of days your menstrual cycle lasts. It is not the same as period length, which is the number of days your period lasts. The day your period starts is considered Cycle Day (CD) 1 -- the start of a new menstrual cycle.
The textbook average for a woman's menstrual cycle length is 28 days, and for period length, 3 to 5 days. Clearly, many of us know firsthand that these textbook stats don't always (or even ever) reflect our unique cycles! Though some of us (OB/Gyns estimate 10%) do have 28 cycles, and regularly, the rest of us have menstrual cycles that are shorter or longer, and can vary by a few days every month.
To calculate menstrual cycle length, you count the number of days from day one of your period (CD 1) up to, and including, the day before the next period starts.
The count does not include the first day of the next menstrual period, because that's Cycle Day 1 of the next cycle.
Period starts June 3rd, and lasts for 5 days. The next period starts June 27.
MENSTRUAL CYCLE LENGTH CALCULATION:
Count the number of days from June 3rd to June 26th. The counting includes June 3rd (CD 1), and also includes June 26th.
The start day of the next period, June 27, is not included as it starts the new cycle.
FINGER COUNTING SIMULATION FOR THIS EXAMPLE (1, 2, 3...)
ANSWER FOR THIS EXAMPLE: Cycle Length = 24 days
Average cycle length, in turn, is calculated by summing up each individual cycle length and dividing by the total number of cycles.