Ovulation Calculator

The first day of your last period:
Average length of your cycles:

This ovulation calculator estimates the ovulation date and fertile window based on the date and average length of your period cycles. To use the calculator, please provide the date of the first day of your last period and your typical period length, then click the "Calculate" button.

What is ovulation?

Ovulation is a phase of the menstrual cycle and plays a major role in pregnancy. The menstrual cycle involves three phases: the follicular phase, ovulation, and the luteal phase. Ovulation occurs about midway through the menstrual cycle. In an average 28-day menstrual cycle, it occurs approximately 14 days before the beginning of the next menstrual period. The days surrounding ovulation are the days that a woman is most fertile, so for those who want to get pregnant, it is important to pay attention to when they are ovulating.

Ovulation is the phase during which the egg is released from the ovaries into the fallopian tube. It stays in the fallopian tube for 12-24 hours, where it can be fertilized (during the luteal phase). Most typically, one egg is released. However, in some cases (1-2%), ovulation may release more than 1 egg within 24 hours of each other; if the two eggs are fertilized by two different sperm, the result is fraternal twins. If the egg(s) is fertilized, it then moves toward the uterus where it may implant and result in pregnancy. If the egg is not fertilized within this period, it disintegrates and is shed along with the uterine lining as a normal part of a woman's period.

Fertile window

The "fertile window" refers to the few days surrounding ovulation during which a woman is most fertile. A viable egg stays within the fallopian tube for 12-24 hours where it can be fertilized by viable sperm. After this period, it can no longer be fertilized.

Sperm can remain viable within the fallopian tube for approximately 5 days. Thus, the fertile window is the ~5 days before ovulation and the 1 day after ovulation occurs. For those who are looking to conceive, having sexual intercourse over this 6-day period surrounding ovulation will maximize the chances of pregnancy occurring. This involves identifying your ovulation day.

How to find the ovulation day

The ovulation day is estimated based on the length of your menstrual cycle. To estimate the length of your menstrual cycle (usually ~23-35 days), determine the number of days from the first day of bleeding for your last period, to the first day of bleeding for your next one. Then, subtract 14 days from the end of your current menstrual cycle to find your approximate ovulation day.


Given that the first day of your last period started on 6/29 and ended on 7/27, your menstrual cycle lasted 28 days. Subtracting 14 days from 7/27, your estimated ovulation date is 7/13.

Signs of ovulation:

Aside from estimating your ovulation day based on the date of your last period, there are some signs to look out for that can help determine when you are ovulating. Note that even if you are ovulating, you may not necessarily exhibit any of these signs, but if you estimate your ovulation day and also notice some of these signs, then you are probably ovulating.

  • Rise in basal body temperature of 0.5 - 1°F during the 12-24 hour period following ovulation. To notice such a small change, it is important to consistently measure your temperature under the same conditions and log your temperature for comparison.
  • Ovulation pain. Some women experience mild pelvic discomfort or cramps before or during ovulation that have been described as twinges or pops.
  • Increase in cervical mucus discharge in the days leaving up to ovulation.
  • Tender breasts
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