Ovulation Calculator

The Menstrual Cycle

A menstrual cycle is counted from the first day of one period to the first day of the next. The cycle has two main phases separated by ovulation:

• Follicular phase: Begins on the first day of your period and ends at ovulation. Its length varies between people and cycles.
• Luteal phase: Runs from ovulation to the start of the next period. It is relatively constant, usually 12-14 days.

Estimating Ovulation

Because the luteal phase is fairly fixed, ovulation is estimated by counting backwards from the expected next period:

Ovulation Day = Next Period Date − Luteal Phase Length

With a 28-day cycle and a 14-day luteal phase, ovulation lands around day 14. A longer or shorter cycle shifts ovulation earlier or later accordingly.

The Fertile Window

Sperm can survive in the reproductive tract for up to about 5 days, while an egg is viable for roughly 24 hours after release. The fertile window is therefore estimated as the 5 days before ovulation plus the ovulation day itself — a 6-day span when conception is most likely.

Next Period Prediction

Each subsequent period is projected by adding your cycle length to the previous period start date. The calendar and table above show several cycles ahead so you can plan.

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