Period Calculator

Predict your next periods, ovulation date, and fertile window based on your last period and average cycle length

First day of last period

Cycle Length 28 days

20 days45 days

Period Duration 5 days

1 day10 days

Predict ahead

Your Period Predictions

Enter your last period date and cycle length, then click Calculate.

Legend: Period days Ovulation day Fertile window

Frequently Asked Questions

The calculator adds your average cycle length to the first day of your last period to predict your next period. For example, if your last period started on 1 May and your cycle is 28 days, your next period is expected around 29 May. Ovulation is estimated at cycle length − 14 days from the start of each period.

A normal menstrual cycle ranges from 21 to 35 days, with 28 days being the average. Cycle lengths can vary month to month by a few days. Consistently very short (<21 days) or very long (>35 days) cycles should be discussed with a healthcare provider.

The fertile window is typically the 5 days before ovulation plus the day of ovulation itself. Since sperm can survive up to 5 days in the female reproductive tract, intercourse in this window can lead to pregnancy. Ovulation usually occurs about 14 days before the next expected period.

Predictions are based on your stated average cycle length and are a good starting point. However, cycles can be influenced by stress, illness, travel, hormonal changes, and other factors. For family planning or medical decisions, always consult a healthcare professional and consider tracking multiple cycles to find your personal average.

If your cycles vary significantly, try using the average of your last 3–6 cycles as your cycle length input. Irregular periods can be caused by PCOS, thyroid conditions, stress, or significant weight changes. A doctor can help identify the underlying cause and provide personalised guidance.

The average menstrual cycle is 28 days, measured from the first day of one period to the first day of the next. However, a cycle is considered normal anywhere between 21 and 35 days. Period duration typically ranges from 2 to 7 days, with an average of about 5 days. Cycle length can vary naturally from month to month, and it is common for a healthy cycle to fluctuate by a few days. Consistent cycles shorter than 21 days or longer than 35 days may warrant discussion with a healthcare professional to rule out an underlying condition.

Many factors can disrupt menstrual cycle regularity: stress — elevated cortisol interferes with the hypothalamic-pituitary-ovarian axis; significant weight changes — both rapid weight gain and loss (including extreme dieting) can suppress ovulation; illness or fever — acute illness often delays the next period; intense exercise — very high training loads (common in endurance athletes) can suppress periods (athletic amenorrhoea); hormonal contraceptives — starting or stopping the pill or other hormonal methods changes cycle patterns; thyroid disorders — both hypothyroidism and hyperthyroidism affect cycle length; PCOS — one of the most common causes of irregular periods. Consult a healthcare professional if irregularity is persistent or sudden.

Ovulation typically occurs approximately 14 days before the next expected period, regardless of cycle length. For a 28-day cycle, this is around day 14. For a 35-day cycle, ovulation is around day 21. Signs of ovulation include a rise in basal body temperature (BBT) (measured first thing in the morning before getting up), changes in cervical mucus (becoming clear and stretchy, like raw egg white), and a surge in luteinising hormone (LH) detectable with ovulation test strips. Tracking these signals alongside period dates gives a fuller picture of your cycle. Digital apps that integrate BBT, LH test results, and cycle dates provide the most reliable ovulation tracking. Consult a healthcare professional for fertility-related decisions.

Polycystic Ovary Syndrome (PCOS) is one of the most common hormonal disorders in people of reproductive age, affecting an estimated 8–13% of the population. It is characterised by elevated androgen (male hormone) levels, polycystic ovaries on ultrasound, and irregular or absent periods due to infrequent or absent ovulation. Cycles in PCOS are often longer than 35 days or entirely absent (amenorrhoea), making period prediction unreliable. PCOS is associated with insulin resistance, weight gain, acne, and excess hair growth. It is one of the leading causes of infertility. If you suspect PCOS, consult a healthcare professional — diagnosis involves blood tests, ultrasound, and clinical evaluation.

Stress — whether physical (illness, extreme exercise) or psychological (anxiety, trauma) — can disrupt the menstrual cycle through its effect on the hypothalamic-pituitary-ovarian (HPO) axis. Elevated levels of the stress hormone cortisol suppress the hypothalamus's release of GnRH (gonadotropin-releasing hormone), which is the master regulator of the reproductive hormone cascade. Without adequate GnRH signalling, the pituitary does not release enough FSH and LH, and ovulation may be delayed or skipped entirely. When ovulation is delayed, the period that follows is also delayed. A single delayed period due to an acute stressor is common and usually self-correcting. Chronic stress leading to repeated missed periods (functional hypothalamic amenorrhoea) requires evaluation by a healthcare professional.

About This Period Calculator

This free Period Calculator predicts your upcoming menstrual periods, ovulation dates, and fertile windows for up to 12 months ahead. Based on your last period date and average cycle length, it helps you plan ahead, understand your cycle, and identify your most fertile days — all without any sign-up or personal data stored.

Standards & References

Wikipedia — Menstrual Cycle

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