Menstrual Calculator

How the menstrual calculator works

A menstrual cycle is counted from the first day of one period (day 1) to the day before the next one starts. If you know when your last period began and how long your cycles usually are, your next periods can be predicted by adding the cycle length again and again:

The calculator lists your next six periods, the expected last day of your current period (from your usual period length), today’s cycle day and your likely ovulation day — ovulation is about 14 days before each period.

What is a normal menstrual cycle?

• Cycle length: for adults, cycles commonly range from about 21 to 35 days. Many people’s cycles vary by a few days from month to month.
• Period length: bleeding usually lasts about 2 to 7 days.
• Changes over time: cycles are often less regular in the first years after periods start and in the years before menopause.

Keep a record of the first day of each period for a few months — that gives you your own average cycle length, which makes every prediction more accurate.

My period is late — what does it mean?

A late period can simply be normal variation, or it can be caused by stress, illness, travel, weight changes, breastfeeding, hormonal contraception — or pregnancy. If you could be pregnant, a home test is most reliable from the day your period is due; the Pregnancy Test Calculator shows that date. If your periods stop for several months or change suddenly, talk to a doctor.

Period and ovulation tracking

A period tracker and an ovulation tracker use the same dates: the first day of your period. If you are trying to conceive, the Ovulation Date Calculator shows your fertile window in detail. If a pregnancy is confirmed, the first day of your last menstrual period is also what doctors use to date it — see the LMP Calculator and the Due Date Calculator.

Period due date calculator

Your next period due date is shown at the top of your result — this menstrual calculator works as a period due date calculator for the next six cycles.