Menstrual Cycle Calculator

ACOG Reproductive Endocrinology & The 4 Cycle Phases

The menstrual cycle is governed by the hypothalamic-pituitary-ovarian (HPO) endocrine axis:

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• Menstrual Phase (Days 1 to 5)**: Progesterone and estrogen levels drop, triggering shedding of the functional endometrial lining.

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• Follicular Phase (Days 6 to 13)**: Pituitary FSH stimulates ovarian follicles; rising estradiol thickens the uterine lining and produces fertile egg-white cervical mucus.

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• Ovulatory Phase (Day of Ovulation ± 1 day)**: A surge in Luteinizing Hormone (LH) triggers the mature Graafian follicle to rupture, releasing the oocyte into the fallopian tube.

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• Luteal Phase (Days 15 to 28)**: The ruptured follicle transforms into the corpus luteum, secreting high levels of progesterone to maintain the endometrium for blastocyst implantation.

The 6-Day Fertile Window

• Research by Wilcox et al. demonstrates that clinical fertility is confined to the 5 days before ovulation plus the day of ovulation itself ([Ovulation - 5 to Ovulation]), peaking 1 to 2 days prior to egg release.

How to Use This Calculator

Enter your Average Cycle Length (Days from Period to Period) (e.g. 28), Period Bleeding Duration (Days) (e.g. 5), Luteal Phase Length (Days) (e.g. 14), Current Day of Your Cycle (Day 1 = First Day of Bleeding) (e.g. 14), and Cycle Pattern Benchmark — select from Custom Cycle Length, Standard 28-Day Cycle (Ovulation Day 14 / Fertile Days 9–14), Short 24-Day Cycle (Ovulation Day 10 / Fertile Days 5–10). Results update instantly — no submit button needed. Use metric or imperial units as labeled. For the most accurate result, measure height and weight in the morning without shoes.

The Formula

Ovulation day = Cycle Length - Luteal Phase

Worked Example

A woman with a 32-day cycle calculates her ovulation as Day 18, establishing her fertile window from Day 13 to Day 18, timing intercourse on Days 16 and 17 to maximize conception odds.

Understanding Your Result

A normal menstrual cycle ranges from 21 to 35 days; the average is 28 days. Cycle length varies by up to 7–9 days between cycles in 20% of women without any pathology. Flow lasting 2–7 days is typical; more than 8 days (menorrhagia) or associated with soaking a pad/tampon hourly may warrant evaluation. Cycle irregularity (outside 21–35 day range) can indicate thyroid dysfunction, polycystic ovary syndrome (PCOS), or hypothalamic amenorrhea — a physician should evaluate persistent irregularity.