Ovulation Calculator
This ovulation calculator estimates ovulation day and a six-day fertile window from the first day of your last period and your cycle length.
How to use
- Enter the first day of your last menstrual period.
- Enter your average cycle length in days. A common default is 28.
- Read the estimated ovulation date and the fertile window. This is a calendar estimate, not contraception or medical advice.
Formula
For a regular cycle, ovulation is estimated as cycle length minus 14 days after the period start. The fertile window is the five days before ovulation plus ovulation day. Irregular cycles make the estimate less reliable.
Example. A period that starts January 1 with a 28-day cycle puts ovulation around January 14, and the fertile window around January 9 through January 14.
Frequently asked questions
How is ovulation estimated?
For a regular cycle, ovulation is often estimated as 14 days before the next period. A 28-day cycle ovulates around day 14.
What is the fertile window?
Sperm can live several days. The fertile window is usually the five days before ovulation plus ovulation day.
Does this work with irregular cycles?
Estimates are less reliable if cycle length varies a lot. Tracking LH or basal temperature is more precise.
Is this birth control?
No. It is an estimate only and is not a method of contraception.
Is my data uploaded?
No. Dates stay in your browser.
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