The first step in figuring this out is to keep track of your period on a calendar for about 3 to 4 months. By doing this, you can determine how many days your cycle is from day one to day one of the next period. Do this for each cycle, then average the numbers. For example, my last four cycles were: Cycle One - 29 days, Cycle Two - 31 days, Cycle Three - 31 days, Cycle Four - 30 days. Add up the number of days, divide by 4 months, and you get an average of 30 days. With that number, you can predict the first day of your next period when adding 30 days to the last. So, if my last period started on February 2, I add 30 days and come up with March 4 for the first day of my next period.
Keeping track of your period is a great idea. Not only will it help you understand your menstrual cycle better, it will help you communicate with your healthcare professional. To track it, just mark in a calendar the days you were on your period in a calendar. Your cycle is the number of days from the first day of your period from one month to the next (not from the last day of one to the first day of the next one). My periods are kind of irregular. So when I have a period one month is not necessarily a good prediction of when I'm going to have it the next month. But I still like to track my period just so I know how long it's been and so I have a record to show my doctor during my yearly gynecologist visit.
Technology makes it really easy for people to track their cycles online. I like mymonthlycycles.com or TheCarrot.com. You can also track your periods using your iPhone with apps like PMS Buddy. If you're not into digital tracking, just use a paper calendar. Make a dot or symbol on the calendar for each day of your period. After tracking for a few months, you should get a pretty good idea of how long your cycles are, and how many days are in between them.
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