Menstrual Cycle Phase Calculator

The menstrual cycle divides into four phases, each driven by a distinct hormonal pattern. The classic 28-day cycle is a statistical average rather than a norm. Anything from 21 to 35 days is considered normal, and the length commonly varies from one month to the next in the same person.

Phase 1, Menstruation (days 1 to period duration): Progesterone and estrogen drop when pregnancy does not occur, triggering the shedding of the uterine lining. FSH begins rising near the end of this phase.

Phase 2, Follicular phase (period end to ovulation-2): FSH stimulates ovarian follicles to develop and produce estrogen. Estrogen rises steadily, thickening the uterine lining. Energy tends to peak in this phase. The follicular phase is the variable part of the cycle: short cycles compress it and long cycles stretch it, which is why ovulation is estimated backwards from the next period rather than forwards from the last one.

Phase 3, Ovulation (approximately cycle day = cycle length minus 14): A surge in LH (luteinizing hormone) triggers the release of the dominant follicle’s egg. This phase lasts 24-48 hours. Basal body temperature rises slightly after ovulation, which is why BBT charting helps confirm (but not predict) ovulation after the fact.

Phase 4, Luteal phase (ovulation+1 to cycle end): The empty follicle becomes the corpus luteum and produces progesterone. This phase is relatively constant at 12-16 days regardless of cycle length. If fertilization occurs, HCG from the embryo maintains the corpus luteum; if not, progesterone drops and the cycle restarts.

Ovulation is estimated as day (cycle length - 14), working backwards from the next period because the luteal phase is the stable part.

The fertile window runs from about five days before ovulation through the day of ovulation itself, and the calculator above shows it as a date range. Five days sounds generous for a 24-hour egg, and it is: the window is set by how long sperm survive in the reproductive tract, not by the egg.

Do not use this as contraception. Calendar methods fail because ovulation is not punctual, and a single cycle where it arrives early is enough. The arithmetic here is for understanding the pattern and for timing conception attempts, not for avoiding them.