How to Calculate Menstrual Cycles

Updated July 20, 2017

Every woman's menstrual cycle is different, but knowing when to expect your period can help determine the best time to get pregnant and help avoid embarrassing mishaps. The average length of a menstrual cycle is 28 days, but some women have reported cycles of 20 to 36 days. Menstrual cycles also can be affected by various factors, such as stress, but women can typically narrow down the time frame for their periods by vigilantly tracking their cycles.

On a calendar, note the exact day that your last menstrual period began as Day 1. Mark an X on that day.

Count each day on the calendar until your next period begins, and mark an X on that day. That date is Day 1 of your latest menstrual cycle.

Go back and count the number of days between the start of your last menstrual cycle and the start of your latest period. That number is the length of your menstrual cycle.


You may have to track your period for several months before determining the average length of your cycle.

Things You'll Need

  • Calendar
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About the Author

Dionne Allyson has been a professional writer and editor for more than 12 years. Her work has appeared in newspapers, specialty magazines, business-to-business newsletters, and on websites such as eHow and LIVESTRONG.COM.