Ovulation Calculator

Find your ovulation day and fertile window from your last period and cycle length, with a 3-month calendar and the dates of your next six cycles.

Luteal phase (days)
Usually 14 days. Change it only if you know yours, for example from ovulation tests or basal body temperature.
ℹ️ Estimate based on an average cycle: ovulation can change from one cycle to the next. Do not use this calculator as a method of contraception.

What it does

Ovulation is the release of an egg, and it happens about 14 days before the next period. This calculator counts back from the end of your cycle to find the ovulation day and the fertile window, the six days that end on the day of ovulation, when pregnancy is possible. It shows a three-month calendar with the period, fertile and most fertile days, the date of your next period, when a pregnancy test becomes reliable and the due date if you conceive in each cycle. Nothing you type leaves your browser.

How to use it

  1. Enter the first day of your last period.
  2. Adjust the average length of your cycle and of your period; change the luteal phase only if you know it.
  3. See the fertile days on the calendar and the dates of the next six cycles.

The arithmetic runs in your browser. Nothing you type is sent anywhere and nothing is stored, so you can use these with real numbers.

Frequently asked questions

How is the ovulation day calculated?
The second half of the cycle, the luteal phase, lasts about 14 days in most women, while the first half varies. So ovulation is counted back from the next period: day 14 in a 28-day cycle, day 18 in a 32-day cycle and day 21 in a 35-day cycle.

What is the fertile window?
It is the six days that end on the day of ovulation. Sperm can survive up to five days, while the egg lives for about a day, so a pregnancy almost always comes from intercourse in these six days, most often in the last three (Wilcox et al., 1995).

How accurate is it?
It is an estimate based on your average cycle. Stress, illness, travel or breastfeeding can move ovulation by several days. Ovulation tests (LH), basal body temperature and changes in cervical mucus help confirm when it really happens.

Can I use it to avoid pregnancy?
No. Calendar-based methods fail often because ovulation is not always on the expected day. If you want to avoid pregnancy, talk to a health professional about a reliable method of contraception.

🤰 Pregnancy Due Date Calculator
NEJM — Wilcox et al. 1995, Timing of sexual intercourse in relation to ovulation
Wikipedia — Fertile window