When and How to Use an Ovulation Calculator or Calendar

Ovulation calculators, charts, and calendars are tools that attempt to predict when you might ovulate. These tools are based on the typical length of your menstrual cycle and the expected window for ovulation to help time sex for pregnancy. Likewise, if you’re taking Clomid (clomiphene) or other fertility medications, these tools can be used to suggest when you’ll be most fertile.

While helpful tools, ovulation calculators are not 100% accurate; other methods of ovulation detection, such as ovulation test kits, are more precise. But an ovulation calculator like the one below can help put you in the fertility ballpark.

Overview

The terms ovulation calculator, chart, and calendar are pretty much interchangeable. They all use the same basic information to predict when you are most likely to ovulate based on past cycles, giving you a projected fertile window to increase your chances of conceiving.

The difference between these tools—calculators, charts, and calendars—is simply the name used and how the information is presented. Various online resources, books, and medical practitioners talk about and offer these tools (and how to do your own calculations) using different terminology but they are all essentially the same.

A calendar tool will most often look like a typical calendar with the estimated fertility windows highlighted in each month. A chart may display the same information in a chart form rather than as a calendar. Additionally, it may be called an ovulation chart simply because you are charting the information. With a calculator, you likely plug your data into a calculator-type field that then provides the expected fertility period.

How It Works

Most simple ovulation predictor tools just ask for the date of the first day of your last period and the average length of your menstrual cycles. If you don’t know your regular cycle length, most calendars will suggest that you use 28 days. This is considered the average but research shows there is actually significant variation from woman to woman (and even cycle to cycle for some women). A "normal" cycle can range from 21 to 35 days.

Then, the calculator (or calendar or chart) will usually assume a luteal phase of 14 days. The luteal phase is the time between ovulation and the first day of your next period. Like cycle length, the length of a "normal" luteal phase can also vary quite a bit and be as short as 10 days or as long as 15. So, if your luteal phase tends toward the lower end of the spectrum, the fertility window provided by a basic ovulation calculator tool might be off by several days.

If you know from prior testing or fertility awareness charting how long your luteal phase averages, be sure to include that information to get a more accurate fertility window from ovulation charting. If not, consider using basal body temperature (BBT) charting to find out.

Next, based on this information, the ovulation predictor tool will determine which cycle days you are most likely to be fertile and may even provide a projected ovulation date. 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 days 17, 18, 19, 20, and 21 of your menstrual cycle.

The calculator figures out your estimated day of ovulation by counting 15 days (the average length of your luteal phase in this example) backward from the 35th day (the average length of your cycle) of your cycle. Some calendars will indicate “possible fertile” days and “most fertile days,” with the most fertile days in our example being days 19 and 20.

In fact, women (and doctors) have been doing these calculations on paper for decades. Just follow the example above but plug in your personal last period start date, cycle length, and luteal phase length (if unknown, use 14 days). For more help on this, see detailed instructions below, and to confirm you are doing it correctly for your particular medical situation, consult with your doctor.

Ovulation Prediction Charting

You can use the calculator above (or one you find elsewhere) or create your own ovulation calendar. Keeping in mind everything above—in particular, that everyone’s cycle is different and that even if three women all have 34-day cycles, they still may ovulate on different days—follow the instructions below. The chart will give you a rough estimate of when you may be most fertile.

To use this chart, you need to:

• Consider the information in the chart as a best-guess estimate. If you want a more accurate prediction of ovulation day, try basal body temperature charting.
• Know the average number of days in your cycle. This is the number of days from when one period starts and the next begins. The best way to estimate this is by charting your periods over several months.
• Remember that you are most fertile one to two days before you ovulate. Your odds of pregnancy during your fertile period increase the closer you are to these optimal few days but pregnancy can still occur in the wider fertility window.
• Understand how to count cycle days. The day you get your period is Cycle Day 1; 15 days later is Cycle Day 15, and so on. The last day of your cycle is the day before you get your next period and is indicative of the length of your cycle.
• Track your fertility window by plotting your cycle on a calendar, then counting back 14 days (or use your luteal phase length, if known) from the expected last day of your next cycle.
• Add three or four days (or more, if you want extra cushion) on either side—especially prior to estimated ovulation day and that's your likely fertility window. The key days are the two or three days prior to ovulation but it's often best to hedge your bets by adding a few extra days.

Accuracy

There is no such thing as a perfectly “accurate” ovulation calendar or calculator. By definition, they are based on averages, and even if you've been charting your periods, your cycle length and ovulation date can vary cycle to cycle.

If your periods tend to be irregular, you can calculate your average cycle length and then give yourself a wider fertility window. Alternatively, you can consider how many days your cycle is when it’s shorter and how many days it is when it’s longer and combine the information.

Luckily, you don't need ovulation prediction tools to be absolutely precise in order for them to be helpful. When trying to conceive, you don't necessarily need a specific date but rather a generously wide range of potentially fertile days.

As long as you have sex frequently throughout the entire fertility window (feel free to add a few days on either end), you don’t need to worry about the calculator telling you you’re fertile slightly too early or too late.

Special Considerations

If you are looking for approaches to identifying your fertile window that are more precise than ovulation calculator tools, consider trying the following:

• Charting your BBT
• Paying very close attention to your body’s ovulation signs
• Tracking cervical mucus changes
• Using ovulation test kits (OPKs)

Fertility Apps

If you want a slightly more accurate ovulation calendar, consider using a fertility or ovulation app on your smartphone to assist you. Fertility apps that ask for your basal body temperature and other fertile signs (as well as allow you to adjust data based on your own observations) are the most accurate. That said, if you want to keep it simple, some programs only ask for your period dates and, as a result, will give you effectively the same information as ovulation calculators.

What’s great about many of these apps, however, is that they take into account your average cycle length over many months and track the information in an accessible record, telling you when you might be fertile right on your phone. But even with these apps, the fertile window dates are still an approximation—and some programs are better than others.

These apps are also useful for when you show up for your annual gynecology exam and the doctor or midwife asks when your last period was. Some apps to try include:

• Clue (for iPhone and Android)
• Glow (for iPhone and Android)
• Ovia (for iPhone and Android)
• Kindara (for iPhone and Android)

Fertility Treatment Cycles

An important caveat to the effectiveness of ovulation calculators is that they are less reliable when you take fertility medications like Clomid (clomiphene) or Femara (letrozole), which can disrupt your natural cycle.

There are “Clomid ovulation calculators” available online but they are generally not accurate enough to depend on. You definitely don't want to miss your most fertile days if you’re in the midst of fertility treatment, so a much better choice is to use an ovulation predictor test kit and/or to have frequent sex all month long.

Using a Calculator as Birth Control

Some people try to use ovulation calculators as a form of "natural birth control." The idea is to limit sex to the predicted infertile days and abstain when the calculator says you'll be fertile. However, as explained above, it's not uncommon for cycles to vary, meaning your average infertile days one month might become your most fertile the days the next.

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

A Word From Verywell

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

Additionally, you may also use an ovulation calendar is for deciding when to start using an ovulation test or ovulation predictor kit. The bottom line is that while the accuracy isn’t foolproof, you can use an ovulation calculator as a guide when trying to time sex for pregnancy.