How Accurate Are Ovulation Calculators?

So you’re looking to get pregnant and want to know the best time to get busy in the bedroom?

The short answer is … when you’re ovulating.

Okay, that’s great and all, but how do you know if you’re ovulating?

You may be tempted to turn to the popular ovulation calculator to help you determine this. But this may be a waste of your time.

Don’t worry – today we are going deep with ovulation calculators to give you the full scoop. 

For starters, ovulation calculators use two things to determine when you’re most likely to ovulate:

  • The first day of your last period
  • The average length of your cycle

Which leads you to wonder … just how accurate are ovulation calculators? And are there any 100% accurate ovulation calculators?

Let’s just say that you don’t want to bet any money on the accuracy of them. 

Read on to learn more.

The 411 on ovulation and the menstrual cycle

Ovulation, or the release of the egg from your ovary, is the key phase of your cycle. It typically happens 12-14 days before the start of your next period, although the exact time can vary.

So how exactly does ovulation happen?

At the beginning of your cycle, your hormone levels are pretty low. This signals the start of your follicular phase, where the pituitary gland produces follicle-stimulating hormone (FSH).

FSH is what triggers the development of follicles in your ovary, one of which matures and produces estrogen. In response to this estrogen, luteinizing hormone (LH) is produced. A “surge” in LH levels triggers ovulation, or the release of the egg from your dominant follicle. This surge typically happens 8-20 hours before ovulation. 

After this, the egg goes on a little journey from the ovary and into the fallopian tube, where it not-so-patiently waits to be fertilized by the sperm. (The egg only lives for 12-24 hours here, so time is of the essence.) 

Meanwhile, the ruptured follicle becomes the corpus luteum, which produces progesterone. 

If fertilization and implantation happen, the placenta forms and takes over progesterone production.

If not, the corpus luteum breaks down, resulting in a drop in hormone levels and the shedding of the uterine lining a.k.a menstruation.

And just like that (cue your Sarah Jessica Parker voice), the cycle starts all over again.

How do ovulation calculators work?

Ovulation calculators look at the first day of your period and your average cycle length to find out when you’re most likely to ovulate.

As we talked about earlier, ovulation usually happens 12-14 days before your next period. So, most ovulation calculators subtract 14 days from your cycle length to predict the day of ovulation. 

So it’ll look a little something like this: 

  • If you have a 28-day cycle – ovulation is predicted to occur 14 days before the next period = 14th day is the predicted day of ovulation (If we are being honest – which we always are with you – ovulation varies quite a bit and doesn’t always abide by the 12-14- days-before-your-next-period “rule”)
  • So, if you started your period on September 1, the 28-day cycle ovulation calculator method predicts (heavy emphasis on the “predicts”) that you will ovulate on September 14

Here’s another example:

  • If you have a 30-day cycle – ovulation is predicted to occur 14 days before the next period = 16th day is the predicted day of ovulation
  • If you started your period on September 1, the 30-day cycle ovulation calculator method predicts you will ovulate on September 16. 

Others will predict your chances of ovulation with a percentage rather than a specific day. 

This is great and all if you have regular cycles, a.k.a. ones that last anywhere from 21-35 days

But if you’re someone who falls outside of that range, have irregular cycles, or don’t know your cycle length, you can see how the rough estimation of an ovulation calculator is probably not going to work for you. 

Are you starting to see where these calculators may have a little too much room for error? There’s more … 

How accurate are ovulation calculators?

The exact algorithm used in most ovulation calculators is a mystery, making it hard to determine the most accurate ovulation calculator.

If you have a regular cycle, then the ovulation calculators are probably going to do a fairly decent job of predicting your ovulation accurately. 

But, there is some evidence that only 14% of women who had a 28-day cycle ovulated on day 14, as the calculator would predict. That’s not very accurate, is it? 

This study found that the most popular ovulation day was day 16 and that there was a wide range of the number of days of ovulation that went from 11 to day 20. 

And similarly, for women having cycles shorter or longer than 28 days, the probability of ovulation is spread across a wide range of days. 

What does this mean?

It means that an ovulation calculator that relies solely upon cycle length would have to identify at least 10 potentially fertile days. To get a 90% probability of getting a window that includes the day of ovulation, you need at least an 8-day window. 

So, crunch those numbers, and you’ll find that ovulation calculators only have a 21% chance of accurately predicting ovulation day. 

This means that ovulation trackers can definitely be wrong. Whomp whomp. 

Another study found that the most common ovulation day is day 15 and that ovulation day was spread out over 10 days, regardless of your cycle length. 

And that was supported by another study that concluded ovulation calculators and apps are not the way to go for people who are TTC


Because they don’t take your hormone levels into account, which means they can’t tell you what is actually going on in your unique, individual body. Rather, they give you a general, blanket estimation that does not reflect the great variation in women’s cycles. 

Plus, they rely entirely on the data that the users give the calculator or app, which is problematic. Of the many women who thought they had a 28-day cycle, only 15% of them actually did. Eek!

In case you need more evidence, we’ve got you. Another study found that 25% of women thought they had a 28-day cycle, but only 12.4% of them did. 

Further, 69% of women have cycles that vary up to 6 days. And 46% vary up to 7 days. 

And, another study found that the estimated fertile window dubbed as such by clinical guidelines was seen to be true for only about 30% of women. The remaining 70% had their fertile window either earlier or later, even though their cycles were regular. 

All this is to say that the information users of these apps and calculators input is not very reliable. 

And again, this is even for people who have regular cycles. Imagine the inaccuracy of these apps and calculators for women who have irregular cycles, experience anovulation, and have conditions like PCOS. Hint: it’s not great.

To make matters even worse, these apps and calculators can’t actually confirm whether you’ve ovulated.

What are more accurate ways to track ovulation?

Now that we’ve pulled back the curtain on ovulation calculators and found them not to be as reliable as we all hoped, it’s time to take a quick look at other options that can give you more accurate predictability of ovulation. 

Cervical mucus

One of the methods by which you can predict ovulation is by looking at your cervical mucus, which changes as your estrogen levels change throughout the month. 

If you’re ovulating, your cervical mucus is like the consistency of egg whites (how appropriate, right?). 

Learn more: Cervical mucus 101: What can your cervical mucus tell you about your body?

Basal body temperature

Another way to learn more about ovulation is by measuring basal body temperature (BBT).

Following ovulation, your progesterone levels rise, which results in your BBT also increasing by 0.5-1 degree Fahrenheit.

The problem with using BBT to predict ovulation is that the temperature change only happens after ovulation. So it’s kind of unreliable to use something to predict ovulation after it’s already happened, right? 

On top of that, BBT can be affected by other things like fever, exercise, and stress.

Follicular scan

If you’re looking for the most accurate way to calculate ovulation, you want a follicular scan.

This tracks follicular growth to tell you when the egg is about to drop from the ovary. 

While this method is stellar for accuracy, where it falls short is the cost. These scans don’t run cheap and they are also not something you can do at home.

Know more: A Closer Look At Follicular Scanning For Ovulation Tracking: Is It Worth All The Inconvenience? 

Ovulation predictor kits (OPKs)

These at-home tests predict when you’re about to ovulate by measuring your LH levels.

But this method is not the most accurate. 

They are based on threshold values, have high chances of cross-reactivity, and are unreliable for women with different LH surge patterns, irregular cycles, and conditions like PCOS. 

Know more: Using Ovulation Tests with PCOS: Can You Trust the Accuracy? 

So what’s the solution? The Inito Fertility Monitor.

The Inito Fertility Monitor tells you the actual numerical values of your hormone levels rather than trying to predict ovulation based on thresholds. It measures FSH, LH, and estrogen levels throughout your cycle to show you your fertile window and progesterone metabolite PdG to confirm whether you’ve ovulated. And it does all this using a single test strip!

How can you increase the accuracy of an ovulation calculator?

So you’ve read up to this point and still want to use an ovulation calculator. 

But if you have regular cycles and know your cycle length, you may find that ovulation calculators can be helpful. 

Just promise us that you won’t take the calculator’s predicted day too seriously. You want to leave a bit of a buffer and keep a margin of a few days on either side of your predicted ovulation day. 

And if you want to use an ovulation calculator, we highly suggest stacking it with an additional method of tracking ovulation. 

If you are experiencing any of the following, you want to consult a doctor:

  • Cycle length of less than 21 days
  • Cycle length greater than 35 days
  • Cycle variation greater than 7 days
  • TTC for more than 1 year (under 35)
  • TTC for more than 6 months (over 35)


  • Knowing when you ovulate is essential if you’re TTC.
  • Fertility is the highest during the fertile window – this includes the four days before ovulation, ovulation day, and one day after ovulation.
  • Ovulation calculators use the first day of your last period and your average cycle length. 
  • Ovulation calculators have been shown to be very limited in accuracy, especially if you have irregular cycles or conditions such as PCOS.
  • These calculators cannot take into account individual hormone levels.
  • Other more accurate ways to predict ovulation include looking at your cervical mucus, tracking your BBT, follicular scans, OPKs, and the Inito Fertility Monitor. 
  • The Inito Fertility Monitor measures both alpha and beta LH levels for increased accuracy and it also measures progesterone metabolite PdG levels so you can confirm ovulation at home. 
  • You can increase the accuracy of ovulation calculators by giving a buffer window and combining them with other tracking methods. 

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