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Ovulation & Fertile Window Calculator

Estimate ovulation date and fertile window from last menstrual period.

Tool inputs stay in your browser · optional AI search sends your query

Good to know: Ovulation typically occurs approximately 14 days before the start of the next menstrual period. Fertile window includes 5 days prior to ovulation plus ovulation day.

How to use it (step by step)

  1. 1Fill in the boxes: “First day of last period (YYYY-MM-DD)”, “Average cycle length (days)”. Example numbers are already there so you can see how it works; replace them with yours.
  2. 2The result updates instantly as you type (on a phone it is below the boxes).
  3. 3The bold line is the main answer. Tap Copy next to any value to copy it.

Words to know

Average (mean) / median
Average = add everything and divide by how many. Median = the middle value when sorted; it is not pulled by extreme values.

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