Period Calculator

Pick the first day of your most recent period, how many days the bleeding lasted and the usual number of days from one period's start to the next. If you are not sure of your cycle length, track a few cycles and use the average.

Menstrual cycle

The menstrual cycle is the monthly series of hormonal changes that prepares the body for a possible pregnancy. Its length runs from the day one period starts up to the day before the following period begins. Cycles in adults typically last 21 to 35 days, and it is normal for them to vary by a few days from month to month; in the first few years after periods begin, cycles of 21 to 45 days are common. A period usually lasts two to seven days.

The cycle has distinct phases. Menstruation is the shedding of the uterine lining when no pregnancy has occurred. During the follicular phase, which overlaps with the period, follicle-stimulating hormone prompts several follicles in the ovary to develop, and rising estrogen rebuilds the uterine lining. A surge of luteinizing hormone then triggers ovulation, the release of an egg, about 14 days before the next period. In the luteal phase that follows, the empty follicle becomes the corpus luteum and produces progesterone to make the lining receptive. If the egg is not fertilized, hormone levels fall and the next period begins.

Because the luteal phase is fairly constant, differences in cycle length mostly come from the follicular phase. That is why the calculator places ovulation 14 days before each predicted period rather than on a fixed cycle day. The fertile window covers the five days before ovulation and the day of ovulation itself.

Next period = last period start + cycle length; ovulation ≈ next period − 14 days

Many things can change a cycle: stress, intense exercise, significant weight change, illness, travel, breastfeeding, hormonal birth control, thyroid disorders and polycystic ovary syndrome. Talk to a doctor or nurse if your periods stop for more than three months when you are not pregnant, come more often than every 21 days or less often than every 35 days, last longer than seven days, are so heavy that you soak a pad or tampon every hour or two, or cause severe pain. Bleeding between periods or after menopause should also be checked.

Tracking periods in a calendar or app helps you notice patterns, plan ahead and give useful information to your health care provider.

Frequently asked questions

Sources

• Office on Women's Health – Your menstrual cycle
• ACOG – Women's Health
• NICHD – Menstruation and Menstrual Problems