Free Ovulation and Fertile Window Calculator

The calculator uses the “Calendar Method” (or Rhythm Method) logic, which is based on standard averages of the menstrual cycle.

  1. Input: You provide the first day of your Last Menstrual Period (LMP) and the average number of days in your cycle (typically 28 days).
  2. Calculation:
    • Next Period: The tool adds your cycle length to your LMP date to predict the start of your next period.
    • Ovulation: Medical standards suggest ovulation occurs approximately 14 days before the start of the next period. The tool subtracts 14 days from your predicted next period date.
    • Fertile Window: Sperm can survive in the body for up to 5 days, while an egg survives for about 12-24 hours. Therefore, the fertile window is calculated as the 5 days leading up to ovulation plus the day of ovulation itself.
  3. Result: The tool displays these calculated dates in a clear, easy-to-read format.

Menstrual cycle. Hormone levels, Ovarian cycle and Endometrium layer. Menstrual, proliferative ovulation and secretory phases. Follicular phase, ovulation and luteal phase.