Period Calculator (Next Period, Ovulation & PMS Predictor)

Predict your next 6 periods, ovulation days, fertile windows, and PMS timing from your recent period start dates, with average cycle length and variability calculated automatically. Everything runs in your browser and nothing is saved.

Menstrual Cycle Basics

Phase	Typical Timing	General Overview
Menstrual Phase	Days 1-5 or so	The uterine lining sheds and is released as menstrual flow, typically lasting 3-7 days depending on the individual.
Follicular Phase	After menstruation until ovulation	A follicle matures in the ovary and the uterine lining gradually thickens. Energy levels often feel more stable during this phase.
Ovulation	About 14 days before the next period	A mature egg is released. The fertile window is concentrated around this point.
Luteal Phase	After ovulation until the next period	Progesterone levels rise, and PMS symptoms are commonly reported during this phase, typically 3-10 days before the next period.

The dates shown by this tool are statistical estimates based on past records and are not a guarantee of your actual period or ovulation dates. If your cycle is consistently irregular or you experience severe symptoms, consulting a gynecologist is recommended.

About the Period Calculator

This period calculator predicts your upcoming periods based on the dates your previous periods started. With two or more entries, it automatically calculates your average cycle length and its variability (the range between your shortest and longest recorded cycles) from the intervals between dates. With only one entry, it uses the cycle length you enter manually.

It also shows your estimated ovulation day (roughly 14 days before your next predicted period), your fertile window (the days around ovulation), and the window when PMS (premenstrual syndrome) symptoms are commonly reported. These estimates rely on the statistical tendency for the luteal phase to stay fairly constant (around 14 days), so your actual cycle can shift with changes in health or circumstances. Everything you enter is processed only in your browser and is never sent to or stored on a server.

How to Use

Enter your recent period start dates Enter as many recent period start dates as you can remember (any order is fine — they are automatically sorted chronologically).
Enter a cycle length if you only have one date If you only have one recorded date, enter your typical cycle length (between 21 and 45 days).
Enter your period duration Enter how many days your period usually lasts to also see estimated end dates alongside the start dates.
Review your results Your next 6 predicted periods, estimated ovulation days, fertile windows, and PMS windows are all displayed together.

Tips for getting more out of it

Entering more period start dates improves the accuracy of your average cycle length. If possible, try entering your last 3 to 6 periods.
If the displayed range between your shortest and longest cycle is wide, predictions for ovulation and your next period will also be less precise — treat them as general estimates.
Stress, sudden weight changes, and certain medications can temporarily disrupt your cycle. If it stays noticeably off for several months in a row, consider consulting a gynecologist.
If you are trying to conceive, combining this tool with basal body temperature tracking or ovulation predictor kits can narrow down your fertile window more precisely than these predictions alone.

Common Uses

Planning trips and events

Check ahead of time whether an upcoming trip, wedding, or important event is likely to overlap with your period.

Planning conception timing

Use the estimated fertile window as a reference when planning timing with a partner while trying to conceive.

Preparing for PMS symptoms

Knowing roughly when PMS symptoms tend to appear can help you plan a lighter schedule or prepare mentally in advance.

Spotting irregular cycles early

Recording several cycles and checking the average and variability can help you notice changes from your usual pattern sooner.

Glossary

Cycle Length: The number of days from the start of one period to the day before the next one begins. The typical average is 28 days, but it varies widely between individuals, commonly ranging from 21 to 45 days.
Ovulation: The day a mature egg is released from the ovary, typically estimated to occur about 14 days before the next expected period, since the luteal phase length is relatively consistent.
Fertile Window: The days around ovulation when conception is most likely, accounting for sperm survival (several days) and the egg's fertilizable window (about 24 hours after ovulation).
PMS (Premenstrual Syndrome): A collective term for physical and emotional symptoms — such as irritability, bloating, and fatigue — commonly reported during the luteal phase before the next period.
Luteal Phase: The period from ovulation until the next period begins. Individual variation is relatively small, typically lasting around 12-16 days.
Basal Body Temperature: Body temperature measured at rest before getting out of bed in the morning. It shifts around ovulation, and gynecologists often recommend tracking it to understand cycle patterns.

Frequently Asked Questions

Yes. In that case, enter your typical average cycle length (usually around 28 days) in the cycle length field. The prediction will be somewhat less accurate than when multiple recorded dates are available.

No. Because sperm can survive inside the body for several days, the fertile window typically begins before ovulation itself and extends to about a day after ovulation, when the egg is still able to be fertilized.

It is shown as roughly 5 days before through the day before your next predicted period. Both the timing and severity of PMS symptoms vary considerably between individuals, so treat this as a general estimate rather than a precise prediction.

No. Everything you enter is processed entirely within your browser, is never sent to a server, and disappears if you reload the page.

Yes, but the more your cycle length varies, the less precise the predictions for ovulation and your next period become. Entering several recorded cycles to see the average and the range of variability, then treating the results as a general guide, is recommended.

Side Note — Where the "28-Day Cycle" Number Comes From

The commonly cited "28-day" average menstrual cycle has long been associated with the lunar cycle (about 29.5 days), and the connection between the moon and menstruation has been discussed since ancient times. In reality, only a portion of people have a cycle of exactly 28 days, and numerous studies report a wide individual range from about 21 to 45 days.

Cycle length is also known to shift with age. It tends to be less stable in the years right after a first period, and often becomes irregular again as menopause approaches. The luteal phase — the time from ovulation until the next period begins — tends to stay relatively constant (roughly 12-16 days) regardless of age or overall cycle length, which is why this tool estimates ovulation by counting backward from the predicted next period rather than forward from a fixed starting point.

The medical concept of PMS (premenstrual syndrome) was only formally established in the 20th century; before that, related symptoms were often dismissed simply as mood swings. Today it is widely recognized as an effect of hormonal changes during the luteal phase on both body and mind, and tracking symptoms and their timing is considered a useful part of everyday health management.

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