Next Period Calculator

How to Use This Calculator

1. Enter the first day of your last period: This is Day 1 of your most recent menstrual cycle — the day bleeding started. Use the date picker to select the date.
2. Enter your average cycle length: Count from Day 1 of one period to Day 1 of the next period. For example, if your last two periods started on March 1 and March 29, your cycle length is 28 days. If you are unsure, track 3 cycles and average them.
3. Enter your period duration: How many days your period typically lasts. Most periods last 3–7 days; 5 days is the average.
4. Click Predict My Next Period: Results show your next period start date, days remaining, ovulation estimate, fertile window, and a full 6-cycle schedule table.

Tip: Your "average cycle length" is the key input — even small differences significantly shift predictions over 6 cycles. If your cycle varies month to month, use the midpoint of your typical range (e.g. if cycles vary from 26–30 days, use 28).

How Period Prediction Works

Period prediction is based on the assumption that your menstrual cycle repeats at a consistent length. The core formulas are:

The ovulation estimate of "14 days before the next period" is based on the luteal phase — the time from ovulation to menstruation — which is consistently 12–16 days in most women (average 14 days), regardless of total cycle length. The follicular phase (from period start to ovulation) varies between women and cycles.

This means:

• In a 28-day cycle: ovulation ≈ Day 14
• In a 32-day cycle: ovulation ≈ Day 18
• In a 24-day cycle: ovulation ≈ Day 10

The fertile window spans 6 days: the 5 days before ovulation plus ovulation day itself. Sperm can survive 3–5 days in the female reproductive tract, meaning intercourse in the days leading up to ovulation can result in fertilisation when the egg is released.

The Four Phases of the Menstrual Cycle

Understanding cycle phases helps interpret the predictions more meaningfully:

• Menstrual Phase (Days 1–5 avg): The uterine lining sheds as a period. Hormone levels (estrogen, progesterone) are at their lowest. Energy may be lower; rest and iron-rich foods are beneficial.
• Follicular Phase (Days 1–13 avg): Overlaps with the menstrual phase. The pituitary gland releases FSH (follicle-stimulating hormone), which stimulates ovarian follicles to develop. Estrogen rises, the uterine lining thickens, and energy gradually increases toward mid-cycle.
• Ovulation Phase (Day 14 avg): A surge in LH (luteinising hormone) triggers the dominant follicle to release a mature egg. This is the most fertile point of the cycle. Some women experience mittelschmerz — mild one-sided pelvic pain — during ovulation.
• Luteal Phase (Days 15–28 avg): The ruptured follicle transforms into the corpus luteum, which produces progesterone to maintain the uterine lining. If the egg is not fertilised, the corpus luteum breaks down, progesterone falls, and the cycle restarts with menstruation. PMS symptoms (bloating, mood changes, breast tenderness) typically occur in the last 1–2 weeks of this phase.

Understanding the Fertile Window

The fertile window is the 6-day span during which conception is possible in a given cycle. It ends on ovulation day because the egg is viable for only 12–24 hours after release. Sperm, however, can live 3–5 days in the cervical mucus and fallopian tubes, so intercourse in the days before ovulation can still result in pregnancy.

Signs of ovulation to look for:

• Cervical mucus changes: Discharge becomes clear, stretchy, and egg-white-like (raw egg white consistency) around ovulation — a reliable natural indicator.
• Basal body temperature (BBT): Resting temperature rises by about 0.2–0.5°C after ovulation and stays elevated until menstruation. Track this daily (before getting out of bed) to confirm ovulation retrospectively.
• Mittelschmerz: Some women feel a brief, mild ache or cramping on one side of the lower abdomen when the follicle ruptures. This is harmless but can help pinpoint ovulation timing.
• LH surge: Ovulation predictor kits (OPKs) detect the LH surge that triggers ovulation, typically 24–36 hours before the egg is released. These are highly reliable for identifying the fertile window.

Important note on contraception: The fertile window shown here is a prediction based on average cycle length — it is not a reliable contraception method. Cycle length varies naturally, and ovulation can shift due to stress, illness, and travel. Do not rely on this calculator for pregnancy prevention.

Irregular Periods — Causes & What to Do

An irregular period is one that consistently falls outside the 21–35 day range, or varies by more than 7–9 days from cycle to cycle. Common causes include:

• PCOS (Polycystic Ovary Syndrome): The most common cause of irregular periods in women of reproductive age. PCOS causes elevated androgens, irregular ovulation, and often longer, unpredictable cycles. Diagnosed by ultrasound, blood tests, and symptom history.
• Thyroid disorders: Both hypothyroidism and hyperthyroidism can disrupt cycle regularity. A simple blood test (TSH) can identify thyroid issues.
• Stress: High physical or emotional stress elevates cortisol, which suppresses GnRH (gonadotropin-releasing hormone) — the hormone that kicks off the cycle. This can delay or skip ovulation and periods.
• Significant weight changes: Both rapid weight loss and obesity affect hormone levels. Low body fat reduces estrogen production; high body fat increases estrogen, disrupting the hormonal cycle.
• Perimenopause: In the years leading up to menopause (typically ages 40–50), cycles become increasingly irregular as ovarian reserve declines and hormone levels fluctuate.
• Hormonal contraception: Starting, stopping, or switching contraception (especially hormonal IUDs, implants, or pills) can cause irregular bleeding for several months.

When to see a doctor: If your periods are consistently irregular (outside 21–35 days), you miss 3 or more consecutive periods, your periods become unusually heavy or painful, or you have other symptoms like excessive hair growth or unexplained weight changes, consult a gynaecologist or endocrinologist.

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