Fertility window calculator

How the numbers work

Most menstrual cycles have two phases. The follicular phase, before ovulation, varies in length from person to person and cycle to cycle. The luteal phase, after ovulation, is more consistent and lasts about 14 days. So this tool estimates ovulation by counting back 14 days from the end of your cycle:

• Estimated ovulation day: your cycle length minus 14. In a 28-day cycle that lands around cycle day 14; in a 32-day cycle it lands around cycle day 18.
• Fertile window: the 5 days before ovulation through ovulation day, a span of 6 days. Sperm can survive in the reproductive tract for several days, and the egg is viable for about a day after release, so conception is possible across this window.
• Two most fertile days: the day before ovulation and ovulation day, when the chance of conception is highest.

Dates are counted from cycle day 1, the first day of your last period. The date for any cycle day is that first day plus the number of days since, so cycle day 14 is 13 days after the start of your period.

This is a calendar estimate based on population averages. It is less accurate if your cycles are irregular, very long, very short, or postpartum, or if you have high stress, thyroid issues, or PMOS (polyendocrine metabolic ovarian syndrome, formerly PCOS), since ovulation timing can shift. Tracking signs such as basal body temperature and cervical mucus, or using ovulation tests, can help you confirm your own pattern. These estimates are for educational information only; they are not medical advice and do not replace testing or guidance from your own practitioner.