Menstrual Cycle & Ovulation Calculator

Understanding Menstrual Cycles & Fertile Windows

Tracking your menstrual cycle provides vital insights into reproductive health, ovulation timing, and period predictions. A standard menstrual cycle spans 28 days (ranging normally between 21 and 35 days) and comprises two primary phases: the follicular phase and the luteal phase.

How Ovulation & Fertility Windows Are Calculated

Ovulation occurs when a mature egg is released from the ovary into the fallopian tube. Because the post-ovulatory luteal phase remains relatively constant at approximately 14 days for most women, ovulation typically occurs 14 days before the start of the next expected menstrual period.

• Ovulation Day: Cycle Length - 14 Days from the start of the last period.
• Fertile Window: Sperm can survive inside the female reproductive tract for up to 5 days, while an unfertilized egg remains viable for 12-24 hours. The fertile window spans 5 days prior to ovulation through 1 day after ovulation.
• Peak Fertility: The 2 days immediately preceding ovulation and the day of ovulation itself.

Worked Example

If your last period started on January 1st and your average cycle length is 28 days:

• Next Period Start: January 29th
• Estimated Ovulation Date: January 15th (Jan 1 + 14 days)
• Fertile Window: January 11th to January 16th
• Peak Fertility Days: January 14th & January 15th