When and How to Use an Ovulation Calculator or Calendar

Ovulation Calculator Accuracy, the Best Apps, and When NOT to Use One

Woman using an ovulation calculator/calendar app on her smart phone
Some of the best ovulation calculators are available as fertility apps. But they still aren't entirely accurate. Yuichiro Chino / Moment / Getty Images

An ovulation calendar or calculator is an online tool that attempts to predict when you might ovulate based on the length of your menstrual cycles. Maybe you’re looking for an ovulation calendar to help you time sex for pregnancy. Maybe you’re taking Clomid and are hoping a calculator can tell you when you’ll be most fertile.

Unfortunately, these kinds of calendars and calculators are not accurate.

Other methods of ovulation detection are much better.

There are times, though, when an ovulation calendar can help.

Also, below, you'll find an easy to use ovulation chart. You can use this to know when you might be most fertile.

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, most calendars will suggest you write in 28 days. This is considered the average. (A normal cycle can range from 21 to 35 days.)

Then, the calculator will usually assume a luteal phase of 14 days. The luteal phase is the time between ovulation and the first day of your period. A normal luteal phase can be as short as ten days or as long as 15.

The nicer ovulation calendars will ask how long your luteal phase is. If you know (from previous basal body temperature charting) how long yours is, be sure to include that information.

Next, based on this information, the ovulation calculator will make a guess at what days you are most likely to be fertile and perhaps the day you may ovulate.

If, for example, you told the ovulation calendar that your average cycle is 35 days, and your average luteal phase is 15 days, it may 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 backward from the 35th day of your cycle.)

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

What Is the Most Accurate Ovulation Calendar/Calculator?

There is no such thing as an “accurate” ovulation calendar or calculator. By definition, they are based on averages.

What you want in an ovulation calendar isn’t an accurate, specific date but a nice generous wide range. That way, you don’t need to worry about the calculator telling you you’re fertile too early or too late. You’ll cover all your bases.

The American Pregnancy Association’s ovulation calendar is easy to use and gives a big range of possible fertile days.

If you want a slightly more accurate ovulation calendar, consider using a fertility or ovulation app on your smartphone. The fertility apps that ask for your basal body temperature and other fertile signs are the most accurate. That said, if you want to keep it simple, some programs only ask for your period dates.

What’s great about these apps is they take into account your average cycle length over many months. They will also tell you when you might be fertile right on your phone.

No searching the web required.

(Even with these apps, the fertile window dates are still a guess, and some programs are better than others.)

These apps are also useful for when you show up for your yearly gynecologist exam, and the doctor asks when your last period was. You’ll know just by checking your phone.

Some apps to try include:

Easy to Use Ovulation Chart

Keeping in mind everything above—that everyone’s cycle is different; that if three women all have 34-day cycles, they may ovulate on different days—here’s an ovulation chart you can use.

The chart can give you a rough estimate of when you may be most fertile.

To use this chart, you need to:

  • Know the average number of days in your cycle: that is the number of days from when one period starts and the next begins.
  • Understand how to count cycle days: the day you get your period is Cycle Day 1. Fifteen days later is Cycle Day 15. And so on...
  • Consider information in the chart as a best-guess-estimate: if you want a more accurate estimate of ovulation day, try basal body temperature charting.
  • Remember you are most fertile one to two days before you ovulate: you can get more details on your odds of pregnancy during your fertile window here.

Also, on the chart below, you’ll find the day to start testing if you’re using an ovulation predictor kit (OPK). Usually, you want to start using it a couple of days before you may ovulate.

What if your periods are irregular?

You can calculate your average cycle length and use that data to use the chart below. Alternatively, you can consider how many days your cycle is when it’s on the shorter side, and how many days it is when it’s on the longer side. Then, combine the information.

Note: Do not use this chart to avoid getting pregnant! If you want to avoid pregnancy, an ovulation chart or calendar isn’t accurate enough.

Days in Your CycleStart Ovulation Kit TestingPotentially Fertile Days
21Day 4Cycle Days 4 to 14
22Day 5Cycle Days 5 to 15
23Day 6Cycle Days 6 to 16
24Day 7Cycle Days 7 to 17
25Day 8Cycle Days 8 to 18
26Day 9Cycle Days 9 to 19
27Day 10Cycle Days 10 to 20
28Day 11Cycle Days 11 to 21
29Day 12Cycle Days 12 to 22
30Day 13Cycle Days 13 to 23
31Day 14Cycle Days 14 to 24
32Day 15Cycle Days 15 to 25
33Day 16Cycle Days 16 to 26
34Day 17Cycle Days 17 to 27
35Day 18Cycle Days 18 to 28

Are your cycles shorter than 21 days or longer than 35 days? This is not considered normal. You should talk to a doctor about your cycles.

I’m Taking Clomid. Should I Use a Clomid Ovulation Calculator?

Ovulation calculators are based on your average cycle. But your cycle on Clomid may be entirely different from your cycle off of fertility drugs. 

There are “Clomid ovulation calculators” online, but they are not reliable. You do not want to miss your most fertile days if you’re in the midst of fertility treatment.

A much better choice is to use an ovulation predictor test kit. Or, an even better choice is to have frequent sex all month long.

Can You Calculate Your Infertile Days With an Ovulation Calculator?

Some people try to use ovulation calculators as a form of "natural birth control." They look for their predicted infertile days and avoid sex when the calculator says they are fertile.

If you really don’t want to get pregnant, don't rely on an ovulation calculator. There’s a significant risk you will have sex when you’re fertile.

First of all, you want to be absolutely sure you’re not fertile, not mostly sure. An ovulation calculator that is only using dates can't tell you with certainty when and when you're not fertile.

Secondly, your cycle may not always be regular. It’s common to have an off cycle here and there. If your cycle is going to be late one month, and you assume you’re infertile on a particular day because you were infertile the previous months on that day, you might be wrong.

Then, you might get pregnant.

Best Way to Use an Ovulation Calendar/Chart

If you’re going to use an ovulation calendar, look at the information as a suggestion. Not fact. If the calendar says you will be ovulating on a particular day, you should consider the week before and after that date as also potentially fertile days.

An ovulation calendar shouldn’t be used to pinpoint an exact day of ovulation. 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.

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 accurate ways to detect ovulation, including

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