Updated December 19, 2014.

An ovulation calendar is an online tool that attempts to predict when you might ovulate based on the length of your menstrual cycles. An ovulation calendar may help you time sex for pregnancy, but its accuracy is poor compared to other ovulation tracking methods (see below for more details).

Some more accurate calendars will also ask for the length of your luteal phase. Still, even with an ovulation calendar like this, it cannot predict ovulation with any certainty.

How Does an Ovulation Calendar Work?

The most basic ovulation calendar will ask you for the date of the first day of your last period, and the average length of your menstrual cycles. If you don’t know what it is, most calendars will suggest you write in 28 days, which is considered to be the average.

Then, it will usually assume a luteal phase of 14 days. (The luteal phase is the time between ovulation and the first day of your menstrual cycle.) The nicer ovulation calendars will ask how long your luteal phase is, and if you know (from previous BBT charting), be sure to include that information.

Then, based on this information, the calendar will make a guess at what day you may ovulate in the coming month.

If, for example, you told the ovulation calendar that your average cycle is 35 days, and your average luteal phase is 15 days, it will display your possibly fertile days as day 17, 18, 19, and 20 of your menstrual cycle. (It figures out your day of ovulation by counting 15 days backwards from the 35th day of your cycle.)

Some calendars will indicated “possible fertile” days and “most fertile days,” with the most fertile days in our example on days 19 and 20.

Can an Ovulation Calendar Detect Ovulation Accurately?

Not really. If you have regular cycles that vary no more than by a day or two, and you know how long your luteal phase is, then you could use an ovulation calendar to time sex for pregnancy.

But most women do not have regular menstrual cycles. Also, most women are unaware of how long their luteal phase is.

If you know how long your luteal phase is, you’ve probably been charting your body basal temperature. BBT charting is a far more accurate way to determine when you’re ovulating.

Best Way to Use an Ovulation Calendar

If you’re going to use an ovulation calendar, consider the information as a suggestion, and not fact. If the calendar says you will be ovulating on a particular day, it’d be better to consider the week before and after that date as potential fertile days.

An ovulation calendar shouldn’t be used to pinpoint an exact day of ovulation, and it should never be used as a method of birth control.

One possible way to use an ovulation calendar is to help decide when to start using an ovulation test or ovulation predictor kit. You should start using the ovulation kit test strips a few days before the day that the ovulation calendar says you may ovulate.

Better Ways to Detect Ovulation

The bottom line is you really shouldn’t rely on an ovulation calendar when trying to time sex for pregnancy. There are more effective ways to detect ovulation, including charting your BBT, tracking cervical mucus changes, or using ovulation test kits.