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.

Have your average cycle length automatically calculated at whenever you update your free period calendar! It's simple! Details below...

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.

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.

Calculating menstrual cycle length ANSWER FOR THIS EXAMPLE: Cycle Length = 24 days

Note: The period length of 5 days does not effect the cycle length calculation. Had this period been 4 days or 6 days, cycle length would still be 24 days.

Average cycle length, in turn, is calculated by summing up each individual cycle length and dividing by the total number of cycles. free period calendar automatically calculates your average menstrual cycle length!

  1. Join MyMonthlyCycles. Go to the free Period Calendar

  2. Track at least your last 2 periods, and as many past periods as you want. Your average cycle length displays.

  3. Each time you track your next period, your cycle length is updated!

  4. Use My History/Quick History, or the Menstrual Cycle Chart, to see your average, shortest and longest menstrual cycle length.

  5. Use My Preferences cycle rules to exclude irregular periods. It's also perfect to exclude the time during which you were pregnant!

  6. Use MyMonthlyCycles WhenMyBaby link to transport your cycle stats to Stickies page, to facilitate using WhenMyBaby's calculators.