Period Calculator

The luteal phase (from ovulation to the start of the next bleed) is governed by the lifespan of the corpus luteum, the temporary endocrine structure that forms in the ovary after the egg is released. In a healthy adult cycle the corpus luteum reliably lives 12 to 14 days before progesterone drops and menstruation begins. This calculator uses the 14-day convention published in Speroff's Clinical Gynecologic Endocrinology and Infertility (the standard reference), which is why ovulation is placed at cycleLength - 14 rather than at the midpoint. A 28-day cycle ovulates on day 14, a 35-day cycle on day 21, and a 24-day cycle on day 10.

Almost all real-world cycle variability lives in the follicular phase, not the luteal phase. The follicular phase (period start to ovulation) depends on how quickly a dominant follicle matures, which is sensitive to stress, sleep, travel, illness, and caloric intake. The luteal phase is hormonally clamped at roughly 14 days. So a person who usually ovulates on day 14 of a 28-day cycle but is under unusual stress this month may ovulate on day 17, pushing the next period to day 31, while the next bleed still arrives 14 days after that delayed ovulation. This is why even people with predictable averages should not treat a prediction as a fixed appointment.

The fertile window is the span around ovulation when intercourse can result in pregnancy. It runs from 5 days before ovulation through 1 day after, giving a 6-day differential and 7 candidate dates. The 5-day lead time is the documented upper bound of sperm viability inside the female reproductive tract (Wilcox et al., NEJM 1995). The 1-day trail is the egg's ~24-hour fertile lifespan after release. Outside that window, conception is biologically improbable. Wilcox's data shows about 84% of pregnancies result from intercourse on one of the 3 days ending on ovulation day itself.

For adults, ACOG describes typical cycles as 21 to 35 days. For adolescents in the early years after menstruation begins, the range is 21 to 45 days. This calculator accepts 21 to 45 days as an input range, which is broader than the usual adult range and does not classify a cycle as normal. Discuss persistent changes, very irregular cycles, or concerning bleeding with a clinician.

No. This is a prediction model for tracking and planning. It is not a fertility awareness method (FAM) and it is not contraception. Calendar-only methods have a typical-use failure rate of about 15 percent per year (Trussell, Contraception 2011), and the failure rate jumps higher in people with variable cycles. Real fertility awareness methods like the Symptothermal Method require daily temperature charting, cervical mucus observation, and trained instruction. If you are using cycle tracking to avoid pregnancy, talk to a clinician about FAM training or pair it with a barrier method.