Period calculator

A menstrual cycle is counted from the first day of one period to the first day of the next. The calculator adds your cycle length to the first day of your last period to predict the next start date, then repeats that for six cycles. The end date is the start date plus your period length minus one day, because the first day counts too.

It needs three things:

• First day of your last period: the day full bleeding started, not spotting.
• Cycle length: the number of days from the first day of one period to the first day of the next.
• Period length: how many days you usually bleed.

The result is a prediction, not an exact date. Cycle length shifts from month to month, so your period may start a few days earlier or later.

Your own data gives the best predictions. Add the first day of each period to the cycle log. Once you have four start dates (three full cycles), the calculator:

• works out your average cycle length from your most recent cycles (up to 12)
• shows your shortest and longest cycle
• shows a likely range for each upcoming period, based on your shortest and longest cycle, when your cycles vary
• flags cycles that vary by more than 7 days, or an average shorter than 21 days or longer than 35

The log is saved only in your browser (localStorage). It's never uploaded, and one button deletes it. It won't show up on another device or in a private window.

According to the NHS and the U.S. Office on Women's Health:

Measure	Typical range
Average cycle	about 28 days
Common range in adults	21–35 days
Period length	2–7 days
Cycle-to-cycle variation	a few days

28 days is only an average; 25- or 32-day cycles are just as common. The American College of Obstetricians and Gynecologists (ACOG) suggests getting checked if your cycle length varies by more than 7 to 9 days from month to month, if cycles are shorter than 21 or longer than 35 days, or if bleeding lasts more than 7 days.

Stress, sudden weight change, intense exercise, travel, illness and stopping hormonal birth control can all affect your cycle. Cycles are often less predictable in the teen years and in the years before menopause.

Your last period started on October 5, 2026, your cycle is 28 days and your period lasts 5 days.

Cycle	Expected start	Expected end	Fertile days (estimate)
1	November 2	November 6	October 14–20
2	November 30	December 4	November 11–17
3	December 28	January 1, 2027	December 9–15

Ovulation is estimated 14 days before the next period, and the fertile window runs from 5 days before ovulation to the day after.

Now say your log shows cycles of 27, 29, 33 and 28 days. The average is 29.25 days, so the calculator uses 29, and because your cycles differ by 6 days it also shows a range for each period. Three cycles out, that range is more than two weeks wide, so dates further ahead are less certain.

For each cycle the calculator also marks the fertile window: the five days before ovulation and ovulation day, plus one day of margin, because sperm can survive up to five days. In a study by Wilcox and colleagues (1995), nearly all conceptions happened within this six-day window.

This is not birth control. Even with regular cycles, ovulation can shift by days, so a calendar estimate can't reliably tell you which days are "safe." If you're trying to conceive, the ovulation calculator goes into more detail, and if your period is late, the pregnancy test timer shows when to test.

The Add to calendar (.ics) button gives you a file that Google Calendar, Apple Calendar and Outlook can import. Events are all-day, and the file is created on your device.

Talk to a gynecologist or your primary care provider if:

• your period regularly lasts more than 7 days or is very heavy (soaking a pad or tampon every 1 to 2 hours, or passing large clots)
• your cycles are shorter than 21 days or longer than 35 days
• your cycle length varies by more than 7 to 9 days from month to month
• you bleed between periods or after sex
• you miss periods for more than 3 months and aren't pregnant
• you bleed after menopause

If you could be pregnant and your period is late, take a pregnancy test first. Your cycle log can help when your doctor asks about your cycle.