To calculate your fertile window, it is necessary to remember that ovulation happens about half way through the cycle, or on about the 14th day of a 28-day cycle.

To determine your fertile window, women with a 28-day cycle should count 14 days from the date they last menstruated (with the first menstruation day counting as Day 1). Ovulation will have occurred about 3 days before and 3 days after that 14th day, and this is considered to be the fertile window.

Use our online calculator to determine when your fertile window is:

How to calculate when your period is irregular

Calculating a fertile window with an irregular cycle is not as accurate, which can be a disadvantage if trying to get pregnant or avoid pregnancy. Because menstruation does not occur at similar times, calculations can be off.

Nonetheless, one way to know approximately when the fertile window occurs during irregular cycles is to track the duration of each cycle for one year, and then subtract 18 days form the shortest cycle and 11 days from the longest.

For example: If your shortest cycle was 22 days, and your longest was 28 days, then 22 - 18 = 4 and 28 - 11 = 17. This means your fertile window will occur between the 4th and 17th days of your cycle.

A more thorough way of knowing when your fertile window will occur when your cycle is irregular is performing an ovulation test. This can be especially beneficial to women who are trying to get pregnant. This test can be purchased at the pharmacy. Women should also be aware of the signs of ovulation, like having egg-white discharge. Learn more about the other signs of ovulation that occur when you are most fertile. /ovulation-symptoms/

For women who are trying to avoid pregnancy, using your knowledge of your fertile window is not an effective contraceptive method. There are much more accurate birth control options available, like using a condom or taking a birth control pill. Learn more about the different birth control options you can consider. /birth-control-options/