Menstrual Cycle Calculator

How it works

next period = period start + cycle length · ovulation ≈ next period − luteal phase (14 days by default) · follicular phase = day 1 to the day before ovulation · luteal phase = ovulation to the day before the next period · fertile window = ovulation − 5 to ovulation + 1 · likely PMS days = the 5 days before the next period · cycle day = days since the period started + 1

A menstrual cycle runs from the first day of one period to the first day of the next and divides into phases: menstruation (the bleed), the follicular phase leading up to ovulation, ovulation itself, and the luteal phase after it. The luteal phase is fairly constant at about 14 days, so ovulation is estimated by counting back from the next expected period; most of the difference between short and long cycles is in the follicular phase. If you know your own luteal phase, enter it under More options. The fertile window covers the five days before ovulation, ovulation day and the day after. Premenstrual symptoms, when they happen, usually fall in the five days before a period. The calculator also shows which cycle day and phase a date falls on — today by default — and projects the dates forward for your next cycles.

Worked example

With a last period on June 1, 2026, a 28-day cycle and a 5-day bleed: menstruation is June 1–5, the next period is expected June 29, and ovulation is around June 15 (cycle day 15, 14 days before the next period). The fertile window runs June 10–16, the luteal phase June 15–28 and likely PMS days June 24–28. June 10 is cycle day 10, in the follicular phase and inside the fertile window.

Sources: ACOG — Your changing body: menstruation and the menstrual cycle · ACOG — Premenstrual syndrome (PMS)

Frequently asked questions