Period & Menstrual Cycle Calculator

A menstrual cycle calculator uses the date of your last period and your average cycle length to predict your next period, ovulation date, and fertile window. Understanding your cycle helps with family planning, recognising patterns, and knowing when to expect hormonal changes throughout the month.

How the Calculator Works

The calculator applies two key formulas:

• Next period = last period date + cycle length
• Ovulation date = next period date − 14 days (the luteal phase is consistently ~14 days in most people)
• Fertile window = ovulation day − 5 days to ovulation day + 1 day (sperm survives 3–5 days; egg survives ~24 hours)

These are statistical averages. Ovulation can vary by several days due to stress, illness, travel, or hormonal shifts. An ovulation predictor kit (LH surge test) confirms the actual ovulation date more precisely.

The Four Phases of the Menstrual Cycle

Phase 1 — Menstrual (Days 1–5)

The cycle officially begins on day 1, the first day of full flow. Estrogen and progesterone drop, triggering the uterine lining to shed. This phase typically lasts 3–7 days. Energy levels are lower, and prostaglandins (hormone-like compounds that cause uterine contractions) can cause cramping.

Phase 2 — Follicular (Days 1–13)

The follicular phase overlaps with menstruation and continues until ovulation. The pituitary gland releases Follicle-Stimulating Hormone (FSH), prompting several follicles in the ovaries to begin maturing. The dominant follicle produces estrogen, which thickens the uterine lining. Rising estrogen also boosts mood, energy, and cognitive sharpness — many people feel their best during this phase.

Phase 3 — Ovulation (Around Day 14)

A surge in Luteinising Hormone (LH) triggers the dominant follicle to release a mature egg. The egg travels down the fallopian tube, where it remains viable for roughly 12–24 hours. This is the peak fertility window. Some people experience mittelschmerz — a mild one-sided twinge or cramp — at ovulation.

Ovulation occurs ~14 days before the next period, not necessarily on day 14 of the cycle. For a 32-day cycle, ovulation is around day 18; for a 24-day cycle, around day 10.

Phase 4 — Luteal (Days 15–28)

After ovulation the empty follicle becomes the corpus luteum, which secretes progesterone. Progesterone prepares the uterine lining for a possible fertilised egg. If implantation does not occur, the corpus luteum breaks down, progesterone drops, and menstruation begins — starting the next cycle. PMS symptoms (bloating, mood changes, breast tenderness) typically appear in the last 5–7 days of this phase.

What Is a Normal Cycle Length?

The average cycle length is 28 days, but a normal range is 21–35 days. Cycle length can vary from month to month by up to 7–9 days and still be considered regular. The luteal phase is the most consistent part (~14 days); the follicular phase accounts for most length variation.

• Short cycle (<21 days) — may indicate early ovulation or a short luteal phase
• Average cycle (21–35 days) — typical and healthy range
• Long cycle (>35 days) — may indicate late or irregular ovulation (PCOS is a common cause)
• Irregular cycles — vary by more than 9 days from cycle to cycle; worth discussing with a doctor

Fertility and Ovulation Tracking

The Fertile Window

Pregnancy can only occur if sperm fertilises an egg within its 12–24-hour viability window. However, sperm can survive in the reproductive tract for 3–5 days. This means the fertile window effectively spans 6 days: the 5 days before ovulation and the day of ovulation itself.

The day before ovulation and the day of ovulation are the most fertile. Having intercourse on these two days maximises the chance of conception.

Signs of Ovulation

• Cervical mucus change — becomes clear, stretchy, and egg-white consistency (raw egg white) in the days leading up to ovulation
• Basal body temperature (BBT) rise — a slight temperature increase of ~0.2°C (0.4°F) occurs after ovulation (progesterone is thermogenic)
• LH surge — detected by ovulation predictor kits (OPKs) 24–36 hours before ovulation
• Mittelschmerz — mild pelvic pain or twinge on one side at ovulation
• Breast tenderness — some people notice slight sensitivity mid-cycle

Ovulation Predictor Kits (OPKs)

OPKs detect the LH surge in urine, typically 24–36 hours before ovulation. They are more accurate than calendar methods for identifying the actual ovulation date, particularly for people with irregular cycles. Digital OPKs also track estrogen levels to identify the wider fertile window.

Frequently Asked Questions

How do I calculate my next period date?

Add your average cycle length to the first day of your last period. For example: if your last period started on May 1 and your cycle is 28 days, your next period is expected around May 29. Use the period calculator above for instant results across multiple upcoming cycles.

When am I most fertile?

You are most fertile in the 2 days before ovulation and on the day of ovulation itself. For a 28-day cycle, this is typically around days 12–14. The broader fertile window spans 6 days (days 9–14 in a 28-day cycle). Use the Ovulation mode in the calculator above to see your personal fertile window.

How accurate is the period calculator?

The calculator gives an estimate based on an average cycle length. It is most accurate for people with regular cycles that vary by fewer than 2–3 days from month to month. For irregular cycles (variation >9 days), tracking actual cycle data over 3–6 months and using OPKs gives more reliable results.

What can delay my period?

Common causes of late periods include: pregnancy, significant stress, extreme exercise, low body weight, thyroid disorders, polycystic ovary syndrome (PCOS), and certain medications. A period more than 7 days late warrants a pregnancy test and, if negative, a conversation with a healthcare provider.

What is the difference between cycle length and period length?

Cycle length is the number of days from the first day of one period to the first day of the next (typically 21–35 days). Period length (or menstrual duration) is how many days you bleed during each period (typically 3–7 days). These are independent measurements.