Ovulation calculator
Enter your last period and cycle length, get your fertile days.
When do I ovulate?
Ovulation typically happens about 14 days before your next period starts, ACOG says — fairly consistent from person to person whatever the cycle length. In a 28-day cycle that is about two weeks after your period starts; in a 32-day cycle, about 18 days after.
What are my most fertile days?
ACOG says pregnancy is possible from about 5 days before ovulation until 1 day after. With a period starting January 1 and a 28-day cycle, that is January 10 to 16, with ovulation on January 15.
How long is my cycle?
Count from the first day of one period to the day before the next one starts. Cycles vary, so use an average of a few months.
How accurate is this?
It is an estimate from your average cycle. Stress, illness and irregular cycles shift ovulation; ovulation tests and tracking basal temperature or cervical mucus confirm it for a given month.
Can I use it as birth control?
Calendar counting alone is not a reliable way to avoid pregnancy. Talk with a health care provider about fertility awareness methods or other birth control.
Does a longer or shorter cycle change the fertile window?
It moves the window but not its length. With a period starting March 1, a 24-day cycle puts ovulation on March 11, a 28-day cycle on March 15 and a 32-day cycle on March 19. The days before ovulation vary; the roughly 14 days after it stay about the same.
Why does the calculator say a cycle must be 20 to 45 days?
Outside that range the simple count-back method is not meaningful, so the calculator asks for a cycle between those lengths.
How the estimate is made
The calculator counts forward from the first day of your last period by the length of your cycle to find the day your next period is expected, then counts back 14 days to estimate ovulation. The fertile window is the 5 days before ovulation, the day of ovulation itself and the day after. The count-back rule rests on the observation, cited by ACOG, that ovulation usually comes about 14 days before the next period, so the length of the first half of the cycle is what varies.
A worked example
Suppose your period starts on March 1 and your cycles average 28 days. The next period is expected on March 29. Counting back 14 days puts ovulation on March 15, and the fertile window runs from March 10 to March 16. With a 32-day cycle the same start date moves ovulation to March 19 and the window to March 14–March 20.
Common mistakes
Assuming ovulation is always on day 14 is the classic one; that is true only for a 28-day cycle. Using the length of your longest or shortest cycle rather than a recent average gives a window that is off by days. Counting the cycle from the end of the period instead of from its first day shifts every date. And if your cycles vary by more than a few days from month to month, the estimate is only a rough guide.
Sources: the American College of Obstetricians and Gynecologists (ACOG) patient pages on fertility awareness and on the menstrual cycle, which give the timing of ovulation relative to the next period and the fertile window. Related: due date calculator, pregnancy week calculator, period calculator and conception calculator.
For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.