Period Calculator
Predict your next period, ovulation date, and fertile window based on your last period and average cycle length. See estimated dates for the next three cycles.
Understanding Your Menstrual Cycle
The menstrual cycle is a coordinated sequence of hormonal events that prepares the body for potential pregnancy each month. It starts on the first day of menstrual bleeding (day 1) and ends the day before the next period begins. While textbooks often describe a standard 28-day cycle, normal cycle lengths actually range from about 21 to 35 days, and many women experience some variation from month to month.
The cycle has two main phases, divided by ovulation. The follicular phase runs from the first day of your period to ovulation. During this phase, the pituitary gland releases follicle-stimulating hormone (FSH), which triggers the ovaries to develop follicles — small fluid-filled sacs, each containing an immature egg. Usually, one follicle becomes dominant and continues to grow while the others stop developing. As the dominant follicle matures, it produces increasing amounts of estrogen, which thickens the uterine lining in preparation for a potential pregnancy.
Ovulation occurs when a surge of luteinizing hormone (LH) causes the dominant follicle to release its mature egg into the fallopian tube. This typically happens about 14 days before the next period — not 14 days after the last one, which is a common misconception. For a woman with a 28-day cycle, those numbers happen to line up. But for someone with a 35-day cycle, ovulation likely occurs around day 21, not day 14.
The luteal phase follows ovulation and lasts until the next period begins. The ruptured follicle transforms into a structure called the corpus luteum, which produces progesterone. Progesterone further develops the uterine lining and raises body temperature slightly. If the egg isn't fertilized, the corpus luteum breaks down after about 14 days, progesterone drops, and the thickened uterine lining sheds — which is your period.
This calculator uses the 14-day luteal phase assumption to estimate ovulation. While 14 days is the average, individual luteal phases can range from 10 to 16 days. If you're tracking ovulation for fertility purposes, methods like basal body temperature tracking, ovulation predictor kits, or cervical mucus monitoring provide more precision than calendar-based estimates alone.
Disclaimer: This calculator provides estimates for informational purposes only. It should not be used as a sole method of contraception or fertility planning. Consult your healthcare provider for personalized medical advice.
Ovulation and the Fertile Window
The fertile window is the span of days during each cycle when pregnancy is possible. It's determined by two biological factors: how long sperm can survive inside the female reproductive tract and how long a released egg remains viable.
Sperm can survive in the cervical mucus and fallopian tubes for up to 5 days under favorable conditions, though 3 days is more typical. The egg, once released during ovulation, remains viable for only about 12 to 24 hours. This creates a fertile window that starts roughly 5 days before ovulation and ends about 1 day after — approximately 6 days total.
The highest probability of conception occurs on the two days before ovulation and the day of ovulation itself. A large study published in the New England Journal of Medicine tracked daily conception probabilities and found that intercourse on the day of ovulation carried about a 33% chance of conception, while intercourse two days before ovulation carried roughly a 29% chance. The probability drops sharply outside this peak window but doesn't reach zero until about 5 days before and 1 day after ovulation.
For this calculator, the fertile window is estimated using the 14-day luteal phase assumption. If your predicted next period starts on day 29, ovulation is estimated at day 15 (29 minus 14). Your fertile window would then run from day 10 through day 16 — five days before ovulation through one day after.
There are important caveats here. Calendar-based fertility prediction is the least reliable method of identifying fertile days, especially for women with irregular cycles. Ovulation doesn't always happen exactly 14 days before a period, and stress, illness, travel, and hormonal fluctuations can shift the timing. More accurate methods include ovulation predictor kits (which detect the LH surge), basal body temperature charting (temperature rises after ovulation), and monitoring cervical mucus changes (fertile mucus becomes clear, slippery, and stretchy, resembling raw egg whites).
If you're trying to conceive, these estimates are a reasonable starting point for timing. If you're trying to avoid pregnancy, calendar predictions alone are not reliable enough to serve as contraception. The World Health Organization classifies calendar-based methods as having a typical-use failure rate of around 12 to 24 percent per year.
Factors That Affect Cycle Length
Cycle length isn't a fixed number for most women. It's influenced by a range of internal and external factors, some of which are temporary and some more persistent.
Stress is one of the most common disruptors. Physical or emotional stress activates the hypothalamic-pituitary-adrenal (HPA) axis, which can suppress the hypothalamic-pituitary-gonadal (HPG) axis responsible for reproductive hormone signaling. The result is often a delayed or missed ovulation, which extends the follicular phase and pushes the entire cycle longer. In severe cases, stress can cause amenorrhea — the complete absence of periods.
Body weight and body fat percentage have a significant impact. The body requires a minimum level of energy availability to maintain reproductive function. Very low body fat — common in elite endurance athletes and women with eating disorders — can suppress estrogen production enough to stop ovulation and periods entirely, a condition called hypothalamic amenorrhea. On the other end, obesity can cause excess estrogen production from fat tissue, leading to irregular cycles, anovulation (cycles without ovulation), and heavier bleeding.
Age changes cycle patterns across a woman's reproductive lifespan. Teenagers often have irregular cycles for the first few years after menarche as the hypothalamic-pituitary-ovarian axis matures. Cycles tend to be most regular during the 20s and 30s. In the years leading up to menopause — a transition called perimenopause, which can begin in the late 30s or 40s — cycles often become shorter initially, then irregular, and eventually stop entirely.
Hormonal contraceptives obviously affect cycle timing. Birth control pills regulate the cycle to a predictable length (typically 28 days with a withdrawal bleed during the placebo week). After discontinuing hormonal contraception, it can take several months for natural cycle patterns to re-establish. IUDs, particularly hormonal IUDs, can make periods lighter or stop them altogether.
Thyroid disorders are another common cause of cycle irregularity. Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can disrupt menstrual patterns. Hypothyroidism tends to cause heavier, more frequent periods, while hyperthyroidism can make periods lighter and less frequent. Polycystic ovary syndrome (PCOS), which affects an estimated 6 to 12 percent of reproductive-age women, is characterized by irregular or absent periods due to hormonal imbalances involving androgens and insulin.
Travel, particularly across time zones, can temporarily disrupt your cycle by throwing off your circadian rhythm, which is linked to reproductive hormone release. Shift work has similar effects — studies show that women who work rotating night shifts have higher rates of menstrual irregularity.
When to See a Doctor About Irregular Cycles
Some variation in cycle length is completely normal. A cycle that runs 26 days one month and 30 days the next is not a cause for concern. But certain patterns deserve a conversation with a healthcare provider.
You should consult a doctor if your cycles are consistently shorter than 21 days or longer than 35 days. Very short cycles can indicate issues with ovulation or progesterone production. Very long or absent cycles may suggest PCOS, thyroid problems, premature ovarian insufficiency, or hypothalamic amenorrhea.
Missed periods in someone who isn't pregnant, breastfeeding, or using hormonal contraception warrant investigation, especially if three or more consecutive periods are missed. This is called secondary amenorrhea and can result from stress, excessive exercise, low body weight, hormonal imbalances, or pituitary or thyroid conditions.
Very heavy bleeding — soaking through a pad or tampon every hour for several consecutive hours, passing blood clots larger than a quarter, or bleeding for more than 7 days — is another reason to seek medical advice. Heavy menstrual bleeding, clinically called menorrhagia, can be caused by fibroids, polyps, clotting disorders, or hormonal imbalances, and it can lead to iron deficiency anemia over time.
Bleeding between periods (intermenstrual bleeding) isn't always serious — it can be triggered by ovulation, hormonal fluctuations, or cervical irritation — but persistent or recurring spotting between periods should be evaluated to rule out infections, polyps, or other conditions.
Severe menstrual pain that interferes with daily activities, doesn't respond to over-the-counter pain relievers, or progressively worsens over time could indicate endometriosis, adenomyosis, or other pelvic conditions. While some cramping during menstruation is normal, pain that has you missing work or school is not something you should just push through.
Sudden changes in cycle pattern after years of regularity also merit attention. If your period was predictable for a decade and suddenly becomes erratic, that shift may reflect a change in health status worth investigating.
Disclaimer: This calculator provides estimates for informational and educational purposes only. It is not a substitute for medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with any questions regarding menstrual health, fertility, or reproductive concerns.
Menstrual Cycle Prediction
Next Period = Last Period + Cycle Length | Ovulation = Next Period − 14 days | Fertile Window = Ovulation − 5 days to Ovulation + 1 day
Menstrual cycle prediction is based on the assumption that the luteal phase — the time between ovulation and the start of the next period — is relatively constant at about 14 days for most women. While the follicular phase (from period start to ovulation) varies based on cycle length, the luteal phase remains more stable. The next period is estimated by adding the average cycle length to the first day of the last period. Ovulation is estimated by subtracting 14 days from the predicted next period date. The fertile window spans approximately 6 days: the 5 days before ovulation (reflecting sperm survival time in the reproductive tract) plus the day after ovulation (reflecting the egg's approximately 24-hour viability). These are estimates based on population averages and individual timing may vary.
Where:
- Cycle Length = Number of days from the start of one period to the start of the next
- Luteal Phase = Assumed constant of 14 days from ovulation to next period
- Fertile Window = Approximately 6-day window centered around ovulation
Example Calculations
Standard 28-Day Cycle
Predicting the next period, ovulation, and fertile window for a 28-day cycle.
Next period = March 1 + 28 days = March 29. Ovulation = March 29 minus 14 days = March 15. Fertile window starts 5 days before ovulation (March 10) and ends 1 day after ovulation (March 16). The period itself lasts from March 1 through March 5.
Longer 32-Day Cycle
Predictions for a woman with a longer-than-average 32-day cycle.
Next period = February 15 + 32 days = March 19. Ovulation = March 19 minus 14 days = March 5. Fertile window runs from February 28 through March 6. With a longer cycle, the follicular phase is extended while the luteal phase stays approximately 14 days.
Frequently Asked Questions
Calendar-based predictions are most accurate for women with regular cycles that don't vary much from month to month. If your cycle is consistently 28 days with only a day or two of variation, predictions will be quite reliable. If your cycle ranges from 25 to 33 days, predictions become rougher estimates. The calculator assumes a 14-day luteal phase, which holds for many but not all women. For fertility planning, consider supplementing calendar predictions with ovulation predictor kits or basal body temperature tracking.
No. Calendar-based cycle tracking alone is not a reliable method of contraception. The typical-use failure rate for calendar methods is 12 to 24 percent per year. Ovulation can shift unexpectedly due to stress, illness, or hormonal fluctuations, and sperm can survive up to 5 days in the reproductive tract. If you want to use fertility awareness for contraception, work with a trained instructor who can teach you to combine multiple indicators — basal temperature, cervical mucus, and calendar tracking — for much better accuracy.
Normal menstrual cycles range from 21 to 35 days in adults and up to 45 days in teenagers during the first few years after menstruation begins. The average is 28 days, but plenty of healthy women have cycles that are consistently shorter or longer. What matters most is consistency — a 33-day cycle that repeats reliably every month is perfectly normal. Large swings from month to month (for example, 24 days one month and 38 days the next) may indicate hormonal fluctuations worth discussing with your doctor.
The luteal phase — the time between ovulation and the start of the next period — is the most consistent part of the menstrual cycle for most women, averaging about 14 days. Unlike the follicular phase, which varies with cycle length, the luteal phase length is relatively stable from cycle to cycle within the same individual. That said, normal luteal phases can range from 10 to 16 days. If you know your personal luteal phase length through tracking, you can adjust the estimate accordingly.
A single missed period can result from stress, illness, travel, significant weight change, or simply a longer-than-usual cycle. Pregnancy is the most common cause in sexually active women of reproductive age. If you're not pregnant and miss three or more consecutive periods, consult a healthcare provider to investigate potential causes such as PCOS, thyroid issues, hormonal imbalances, or hypothalamic amenorrhea. An occasional skipped period is usually not concerning, but a persistent pattern warrants evaluation.