How to Calculate Menstrual Cycles Manually. Get a calendar. This can be either a paper calendar or one on a computer, phone, or tablet. Mark the FIRST day of your LAST period with an "X". This is day #1. Count forward however many days your menstrual cycle normally is before you start. If you don't know, then just count 28 days.
Calculate next periods based on your past menstrual cycles (last period, average cycle length). Planning a vacation, special date, check if you may have your period on your wedding, and more, with the period calendar which displays your monthly cycles.