On the first day of your next period, mark it on your calendar, just a little number 1, on the day it starts...and put a 2 on the next day and 3 on the third day and so on and so on, keep on counting the days on the calendar until your period starts AGAIN...on that day start over at number one and voila, you know how long your cycle is...if you mark 28 days on the calendar before you start another period, it's a 28 day cycle...if you want to know when your ovulating, thats a little trickier, it's usually around day 14 for a 28 day cycle, but thats not gospel...
umm its from the first day of bleeding one month to the first day of the next. Like as soon as you see blood count that as the first day. I think that's correct. Its what i did