Period Calculator

This calculator estimates future period days and the most probable ovulation days, laid out as a calendar. Period days are the days bleeding and discharge occur. The most probable ovulation days are the days you are most likely to release an egg.

The menstrual cycle

The menstrual cycle is the series of changes a body goes through in preparation for a possible pregnancy. It usually begins between 12 and 15 years of age and continues until menopause, which happens on average at 52. The cycle is counted from the first day of one period to the first day of the next, and it is driven by hormones rising and falling.

Cycle length varies from person to person. A cycle is considered regular when the longest and shortest cycles differ by fewer than 8 days. The average is 28 days, though anything from 21 to 35 is common in adults, and cycles in the first few years after menarche and the years approaching menopause are more variable than that.

During the cycle the lining of the uterus thickens and an egg is produced. The egg is released from the ovaries in a process called ovulation, which is when you are most fertile. If the egg is not fertilised, pregnancy cannot happen, the lining sheds as a period, and the cycle restarts.

A period is the regular discharge of blood and mucosal tissue through the vagina, and it usually lasts between 2 and 7 days. It falls in the menstrual phase at the start of the cycle, which begins when the egg from the previous cycle has not been fertilised. Periods stop during pregnancy, usually do not return during the early stages of breastfeeding, and stop permanently at menopause, generally between 49 and 52, defined as a full year without vaginal bleeding.

How the dates are worked out

Everything here comes from three inputs: the first day of your last period, how long that period lasted, and your average cycle length.

Next period = first day of last period + cycle length

The ovulation estimate uses the part of the cycle that is stable. The luteal phase, running from ovulation to the next period, holds at roughly 14 days regardless of how long the whole cycle is. The follicular phase before ovulation is the part that stretches and shrinks.

Ovulation day = next period date - 14 days

where the fertile window shown is the two days either side of that, giving five most probable days.

On a 28-day cycle starting 9 August, the next period is due 6 September, ovulation falls on 23 August, and the most probable days run from 21 to 25 August. On a 32-day cycle ovulation moves to day 18 rather than day 14, because the extra four days land in the follicular phase and not the luteal one.

This is the single most useful thing to understand about cycle timing. Counting 14 days forward from your period only works on a 28-day cycle. Counting 14 days back from the next one works on any length.

The four phases

Phase	Runs	What happens
Menstrual	days 1 to 5, roughly	The uterine lining sheds. Hormone levels are at their lowest.
Follicular	day 1 to ovulation	Follicles mature and oestrogen climbs, rebuilding the lining. This is the phase whose length varies.
Ovulation	about 14 days before the next period	An egg is released and survives 12 to 24 hours.
Luteal	ovulation to the next period	Progesterone rises. Holds near 14 days in most people.

The menstrual phase overlaps the start of the follicular phase, which is why the numbers in the table add to more than the cycle length.

The fertile window

The egg survives 12 to 24 hours after release, but sperm survive up to five days in the reproductive tract. That asymmetry is what makes the fertile window roughly six days long: the five days before ovulation plus the day itself. Intercourse two days before ovulation carries a higher chance of conception than intercourse on the day.

The calculator shows five most probable days rather than six, centred on the estimated ovulation date, because the estimate itself has error in it. A wider window around an uncertain centre is more honest than a precise one around a guess.

Signs that ovulation is near are worth more than a calendar. Cervical mucus becomes clear and stretchy, like raw egg white, in the days beforehand. Basal body temperature rises about 0.3°C after ovulation, which confirms it happened but only in hindsight. Ovulation predictor kits detect the luteinising hormone surge 24 to 36 hours before release, which is the most useful advance warning available at home.

Using this to avoid pregnancy

Do not. Calendar methods have a typical-use failure rate of roughly 12% to 24% a year, meaning up to a quarter of people relying on them conceive within twelve months. The reason is straightforward: cycle length varies, ovulation moves with it, and sperm survive long enough that an early ovulation catches out a calendar built on last month's dates.

Fertility awareness methods that combine temperature, cervical mucus and cycle tracking perform considerably better than dates alone, but they need training and daily consistency. If pregnancy would be a problem, use a method designed to prevent it.

When cycles are irregular

An occasional cycle that runs long or short is normal. Stress, illness, travel, hard training, weight change and poor sleep all shift timing, usually by delaying ovulation rather than shortening the luteal phase.

Some patterns are worth raising with a doctor. Cycles consistently shorter than 21 days or longer than 35. A difference of more than 8 days between your longest and shortest cycles. Bleeding for more than 7 days, or soaking through a pad or tampon every hour for several hours. Bleeding between periods. Periods that stop for three months without pregnancy. Pain severe enough to stop you doing normal things, which is not something to put up with quietly.

Polycystic ovary syndrome, thyroid disorders, endometriosis, uterine fibroids and perimenopause all show up as changes in cycle pattern, and all are more manageable when caught early.

What the estimate assumes

The dates here assume your cycles keep the average length you entered and that ovulation lands 14 days before each period. Both are reasonable starting assumptions and neither is a certainty. Real cycles vary by several days in either direction for most people, and the further into the future you look, the more that variation compounds. The sixth cycle in the table carries considerably more uncertainty than the first.

Tracking your own cycles for three to six months gives a far better average than any default. If your recorded lengths vary widely, use the shorter figures when the question is whether you might be fertile, and the longer ones when the question is when a period is due.

What counts as a normal cycle

The American College of Obstetricians and Gynecologists puts the usual adult cycle at 21 to 35 days, measured from the first day of one period to the first day of the next, with bleeding lasting 2 to 7 days and the heaviest flow in the first three. Cycles in the first years after menarche run longer and less predictably, and roughly 90% fall between 21 and 45 days. By the third year after the first period, 60% to 80% of cycles have settled into the adult range.

ACOG treats the menstrual cycle as a vital sign in adolescents, alongside pulse and blood pressure, because a pattern that sits well outside those ranges is often the first visible sign of something else. That is the useful way to read a tracking app: the average matters less than a change from your own baseline.

When the pattern is worth a conversation

Some patterns are worth raising with a clinician rather than tracking through. Cycles shorter than 21 days or longer than 35 in an adult, bleeding that lasts more than 7 days, soaking through a pad or tampon every hour for several hours, bleeding between periods or after sex, and three or more missed periods in a row all fall into that group.

None of those means something is wrong on its own. Thyroid conditions, polycystic ovary syndrome, fibroids, stress, weight change and hormonal contraception all shift the calendar, and each has a different answer. A prediction from any calculator, this one included, describes an average rather than a diagnosis.