Menstrual Cycle Calculator

Predict your next period by entering the first day of your last period and your average cycle length. Our calculator adds your cycle length to your last period date to predict when your next period will arrive — and continues predicting for up to 12 cycles ahead.

Track your menstrual cycle length by recording the first day of consecutive periods. Most cycles range from 21 to 35 days. A 28-day cycle is average, but 'normal' is whatever is consistent for you. Cycles that vary by more than 7 days may indicate hormonal changes worth monitoring.

Your menstrual cycle calculator also predicts your ovulation date and fertile window. Ovulation typically occurs 14 days before your next period. The fertile window spans 5 days before ovulation through ovulation day — a total of 6 days when pregnancy is possible.

If your periods are irregular, calculate your average cycle length using 3–6 recent cycles. Add up the days and divide by the number of cycles. Enter this average for the most accurate predictions. For highly irregular periods, consider tracking basal body temperature alongside this calculator.

Your menstrual cycle has four phases: Menstrual (days 1–5), Follicular (days 6–13), Ovulation (around day 14), and Luteal (days 15–28 for a 28-day cycle). Understanding each phase helps you predict symptoms, energy levels, and fertility throughout the month.

A menstrual cycle calculator predicts your next period by adding your average cycle length to the first day of your last period (LMP). The math is simple and timeless: if your period began on the 1st and your cycle is 28 days, the next one is expected 28 days later.

The tool then estimates ovulation by counting back a fixed luteal phase of about 14 days from that predicted next-period date. According to the American College of Obstetricians and Gynecologists (ACOG), the luteal phase stays fairly constant even when total cycle length varies.

The calculator applies this same logic across several cycles to build a forward calendar:

• LMP — the anchor date the whole prediction is built from
• Cycle length — the gap between two period start days
• Luteal offset — the roughly 14-day window used to place ovulation

Because it uses your averages rather than real-time hormone data, treat every date as an estimate, not a guarantee.

To use the calculator, enter your last period's first day, choose your average cycle length, then read the predicted dates — no account or personal data storage needed.

Work through a real example. Suppose your last period started May 3 and your average cycle is 30 days:

• Next period: May 3 + 30 days = June 2
• Ovulation: about 14 days before June 2 = around May 19
• Fertile window: the roughly 6 days ending on ovulation day, so about May 14 to May 19

Follow these steps for the most reliable result:

• Log the first day of full flow, not spotting, as your start date
• Average your last 3 to 6 cycles before entering a cycle length
• Re-run the calculator each month with your newest period date

The U.S. Office on Women's Health notes that a few months of tracking sharpens accuracy far more than any single estimate.

The most common tracking mistake is counting cycle length from the end of one period instead of from the first day of each period — a cycle always runs start-day to start-day. That single error can throw every prediction off by several days.

Watch for these frequent slip-ups:

• Logging spotting as day 1. Light pre-period spotting is not the start of flow; the CDC and ACOG both define the cycle from the first day of true bleeding.
• Using one cycle as your average. A single month is a snapshot; average several cycles instead.
• Trusting calendar dates during teen years or perimenopause. Cycles are naturally more variable then, so predictions drift.
• Treating the fertile window as birth control. Calendar timing alone is a low-reliability contraceptive method.

When in doubt, keep a simple log and compare it against the calculator, then discuss persistent irregularity with a healthcare provider.

See a healthcare provider when your cycles are consistently shorter than 21 days, longer than 35 days, or vary by more than 7 to 9 days month to month, since these patterns can point to a hormonal or medical cause rather than normal variation.

The American College of Obstetricians and Gynecologists (ACOG) and the National Institutes of Health (NIH) flag several patterns worth medical attention:

• Missing three or more periods in a row when you are not pregnant
• Bleeding between periods or unusually heavy flow that soaks through protection hourly
• Cycles that suddenly change in length, flow, or symptoms
• Periods lasting longer than 7 days on a recurring basis

Conditions such as polycystic ovary syndrome (PCOS), thyroid disorders, and endometriosis commonly disrupt cycle timing. A calculator can help you document the pattern, but it cannot diagnose the cause — bring your tracking history to the appointment.

Your cycle length is rarely fixed for life — it shifts predictably with age and hormonal stage. Understanding this helps you interpret why the calculator's estimates may need adjusting over time.

According to the U.S. Office on Women's Health and the World Health Organization (WHO), cycles follow a broad arc:

• Teen years (after menarche): cycles are often long and irregular for the first few years as ovulation becomes established
• Twenties to late thirties: cycles tend to be most regular and predictable
• Perimenopause (typically forties): cycles may shorten, lengthen, or skip as hormone levels fluctuate
• Menopause: defined by the CDC and NIH as 12 consecutive months without a period

During the more variable stages, re-average your recent cycles frequently and lean on the calculator as a rough guide rather than a precise schedule. Pregnancy, breastfeeding, and hormonal birth control also reset the pattern entirely.

A cycle calculator can help you time intercourse for conception, but it is a weak tool for preventing pregnancy on its own. It shows your estimated fertile window — the roughly 6 days ending on ovulation day when conception is possible.

For those trying to conceive, ACOG notes the highest chance of pregnancy falls in the 2 to 3 days before ovulation and on ovulation day itself, and once conception happens you can switch to our pregnancy due date calculator to estimate your delivery date. Timing intercourse across the fertile window improves your odds.

For those avoiding pregnancy, be cautious:

• Sperm can survive up to about 5 days, so the fertile window is wider than a single day
• Ovulation timing shifts with stress, illness, and travel, moving the window unexpectedly
• Calendar-based fertility awareness methods have higher typical-use failure rates than most other contraceptive methods

If preventing pregnancy is the goal, the CDC recommends pairing awareness methods with a more reliable option or consulting a provider.