How do I calculate my next period date?

Your next period is estimated by adding your average cycle length to the first day of your last period. For a 28-day cycle, if your last period started on the 1st, your next period is expected around the 29th. Cycle length can vary from 21–35 days and is perfectly normal.

When am I most fertile during my cycle?

You are most fertile during the 5 days before ovulation and on ovulation day itself — this is called the fertile window. Ovulation typically occurs 14 days before your next expected period. For a 28-day cycle, that means ovulation around day 14.

What is a normal menstrual cycle length?

A normal cycle ranges from 21–35 days, with 28 days being the textbook average. Cycle length can vary month to month, even in healthy individuals. Consistently short (under 21 days) or long (over 35 days) cycles should be discussed with a healthcare provider.

How accurate are period predictions?

Period predictions are estimates based on your reported average cycle length. They become more accurate when you track several consecutive cycles to find your true average. Stress, illness, travel, and hormonal changes can all shift your cycle.

Why is my period late?

Many factors can delay a period beyond the predicted date: stress, significant weight change, intense exercise, illness, thyroid issues, or pregnancy. A delay of 1–2 days is very common. If your period is consistently more than 7 days late, see a healthcare provider.

How many days is a typical period?

A typical period lasts 2–7 days, with 5 days being average. Blood loss averages 30–80 mL per cycle. Periods shorter than 2 days or longer than 7 days, or unusually heavy flow, may warrant evaluation by a doctor.

What causes irregular periods?

Irregular periods — cycles consistently outside the 21–35 day normal range or that vary by more than 7–9 days between cycles — have many possible causes. Polycystic ovary syndrome (PCOS) is the most common hormonal cause, affecting 6–12% of women. Other causes include thyroid disorders (hypo- or hyperthyroidism), significant weight changes, excessive exercise, high stress, hormonal contraceptives, and perimenopause. A healthcare provider can evaluate irregular cycles with hormone testing to identify the underlying cause.

What is the difference between the follicular and luteal phase?

The menstrual cycle has two main phases. The follicular phase begins on Day 1 of menstruation and ends at ovulation — typically days 1–14 in a 28-day cycle. During this phase, follicle-stimulating hormone (FSH) stimulates ovarian follicles to mature, estrogen rises, and the uterine lining thickens. The luteal phase runs from ovulation to the start of the next period (days 14–28). After ovulation, the follicle becomes the corpus luteum and secretes progesterone, which prepares the uterus for a potential fertilized egg. If fertilization does not occur, progesterone drops, triggering menstruation.

Period Calculator — Menstrual Cycle & Ovulation Dates

Predicts your next 6 periods, ovulation date, and fertile window from your last period and menstrual cycle length.

How to Use the Period Calculator

This period calculator predicts your next six periods, your estimated ovulation date, and your fertile window — the days when pregnancy is most likely. Enter the first day of your last period, your average cycle length (21–35 days), and how many days your period typically lasts. Most women have cycles between 24 and 35 days; a 28-day average is used if you are just starting to track.

How Menstrual Cycle Prediction Works

Period prediction is based on a straightforward formula: your next period is expected to start approximately one full cycle length after the start of your last period. For a 28-day cycle, that is 28 days after your last period began. Subsequent periods are calculated by adding additional cycle lengths.

Ovulation and the Fertile Window

Ovulation typically occurs 14 days before the next period — not 14 days after the last one. Since sperm can survive up to five days in the reproductive tract, the fertile window begins five days before ovulation and includes ovulation day itself, for a total of six days. Identifying this window is the foundation of both natural family planning and fertility tracking.

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Normal Menstrual Cycle Lengths

The American College of Obstetricians and Gynecologists (ACOG) defines a normal cycle as 24–38 days. The classic "28-day" cycle is a statistical average, not a requirement. Shorter cycles (21–23 days) are common in teenagers and women approaching menopause. Longer cycles (35+ days) may indicate hormonal variations including polycystic ovary syndrome (PCOS). Tracking several cycles gives you a personal baseline far more accurate than any population average. For a fuller picture of your health, our macro calculator can help you understand how nutrition supports hormonal balance and cycle regularity.

Why Cycles Vary Month to Month

Even in healthy individuals, cycle length can shift by several days each month. Common causes include stress, significant changes in body weight, intense exercise, travel across time zones, illness, and changes in sleep patterns. The calculator uses your reported average; your actual cycle may be shorter or longer in any given month.

When to Consult a Doctor

See a healthcare provider if your cycles are consistently shorter than 21 days or longer than 35 days, if you miss three or more consecutive periods (and are not pregnant), if your period lasts more than 7 days, if bleeding is very heavy (soaking a pad or tampon every hour), or if you experience severe pain. These may indicate treatable conditions such as thyroid disorders, PCOS, uterine fibroids, or endometriosis.

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Period Tracking Tips

Track for 3 months minimum — your true average cycle length emerges after several cycles
Note cycle day 1 accurately — this is the first day of full flow, not spotting
Log symptoms — spotting, cramping, and flow changes help identify patterns
Use BBT for ovulation confirmation — basal body temperature rises slightly after ovulation
LH strips provide a direct ovulation signal — urine-based tests detect the LH surge 24–36 hours before ovulation
Monitor body composition over time — tools like our waist-to-hip ratio calculator can help track physical changes that may correlate with cycle regularity

PCOS and Irregular Cycle Prediction

Polycystic ovary syndrome (PCOS) affects 6–12% of women of reproductive age and is a leading cause of irregular cycles and anovulation (cycles without ovulation). Women with PCOS often have cycles ranging from 35 to 90+ days, making prediction calculators less reliable. PCOS is characterized by elevated androgens (testosterone), multiple small ovarian follicles that fail to release an egg, and often insulin resistance.

For women with PCOS, calendar-based prediction tools like this calculator are less accurate than hormonal tracking methods — LH strips and basal body temperature monitoring provide more cycle-specific data. If your cycles are consistently over 35 days, see a healthcare provider; PCOS is treatable and managing it improves both cycle regularity and long-term metabolic health. Because PCOS is often linked to insulin resistance and body composition, our lean body mass calculator and ideal body weight calculator can help you track relevant health metrics alongside cycle monitoring.

How Hormonal Contraceptives Affect Your Cycle

Combined oral contraceptive pills (estrogen + progestin) suppress ovulation and create a highly regular, predictable withdrawal bleed — not a true menstrual period. Progestin-only methods (mini-pill, hormonal IUD, implant, injection) can cause irregular spotting or absent periods. After stopping hormonal contraceptives, it typically takes 1–3 months for natural cycle patterns to return, though it can take up to 6 months in some cases.

This calculator is designed for natural cycle prediction. If you are using hormonal contraception, cycle length predictions from this tool will not apply to your withdrawal bleeds, which are determined by your pill schedule rather than natural cycle length. For those tracking early pregnancy after stopping contraception, our hCG doubling time calculator can help interpret early pregnancy test results.

Sources & References

Menstrual Cycle: What's Normal, What's Not — Mayo Clinic
Menstruation and the Menstrual Cycle — Office on Women's Health
Your Menstrual Cycle FAQ — American College of Obstetricians and Gynecologists

Period Calculator