How Do You Use a BBT Chart to Pinpoint Ovulation?

There are several tools that can help you figure out when you’re ovulating so you can maximize your chances for getting pregnant. One option is a basal body temperature (BBT) chart to track your basal body temperature.

BBT is the baseline body temperature you get as soon as you wake up in the morning — before you get out of bed, talk or even sit up. Your BBT rises about half a degree shortly after ovulation. Using a BBT chart can help you keep track of your body’s temperature changes and detect when you’re likely to be ovulating.     

How exactly does it work — and is it accurate? If you’re thinking about using a BBT chart to track your ovulation, here’s everything you need to know.

What is a BBT chart and how does it work?

A BBT chart tracks fluctuations in your basal body temperature (BBT) that occur within your cycle over the course of several months. A slight increase in your BBT signals that you’ve ovulated. Over time, having that information mapped out on a chart can help you determine when you’re most likely to get pregnant.

You can track your BBT using an old-school paper chart. But you can also use a smart thermometer to log the data into an app that’ll make a chart for you.

How do you use a BBT chart to detect ovulation?

Using a basal body temperature chart is very easy, but it requires a little bit of dedication. You’ll need to use a digital BBT thermometer (a special type of thermometer that tracks temperature changes to a tenth of a degree) to take your temperature every morning before you do anything else and log the results on a paper chart or in an app. Here’s how:

• Take your temperature first thing in the morning. Use a digital BBT thermometer to take your temperature as soon as you wake up. Do it while you’re still lying down before you do anything else — even sitting up, talking or taking a sip of water.

• Record your temperature. Plot your temperature on the BBT chart or record the data in a BBT app. (Make sure you’ve gotten at least three hours of uninterrupted sleep for the most accurate reading.) If you’re using a smart BBT thermometer, your temperature might get stored on your phone automatically. Your BBT rises about half a degree immediately after ovulation, so if the increase has held steady for three days or more, it’s safe to assume that you’ve ovulated.

• Look for a trend. After a few months you should start to detect a pattern of temperature highs and lows. You can use that information to predict when you’ll ovulate during your next cycle.  

• Have sex when you’re most fertile. Ovulation usually happens a day or two before your BBT rises. (For instance, if your BBT increase happens on day 16 of your cycle each month, you’re probably ovulating on day 14 or 15.) Plan to have sex on the days that you’re ovulating, and give yourself some extra insurance by having sex one or two days before and after ovulation too. (Sperm can hang around for a few days waiting for the egg to show up.)

How accurate is a basal body temperature chart for pinpointing ovulation?

BBT charts or apps can help you get a better sense of when you’re likely to be ovulating, but they’re far from perfect. Findings suggest that accuracy can range anywhere from 76 to 88 percent.  

Why aren’t the charts more precise? There are lots of things that can affect your basal body temperature other than ovulation, including stress, illness or even drinking alcohol. Anything that messes with normal sleep patterns — from shift work and traveling across different time zones to plain old insomnia — can have an impact too. So noticing a rise in your BBT doesn’t automatically mean that an egg was just released. It’s possible to ovulate without your BBT going up, too.

There are also logistics to consider. You’ll need to take your temperature every single morning for at least a few months to detect a pattern. Remembering to do that the second you wake up before doing anything else might be tough.  

How much do BBT charts cost?

Free charts are easy to find online, but you can also pick one up from your OB/GYN or a fertility clinic. Standalone digital BBT thermometers without an app are around $15.

Prefer to go the smart route? You can buy a digital BBT thermometer that syncs up with an app on your phone for $30 to $40.

How long does it take to use one?

Taking your temperature with a BBT thermometer takes about five minutes. But it’ll take a few months of daily temperature tracking before you’ll be able to detect a pattern and predict when you’ll ovulate.

Are there any downsides to using a BBT chart to find out when you’re ovulating?

Using a BBT chart is safe, simple and inexpensive. But it’s not necessarily the best method for tracking your ovulation. With an accuracy rate of 76 to 88 percent, BBT charts are considerably less reliable than ovulation test strips.

They’re also slower. Tracking your BBT only gives you a heads-up about ovulation after it's already happened, and you’ll need a few months’ worth of data before you can predict when you’re about to ovulate so you know the best time to have sex. In other words, they’re not the best tool if you want to maximize your chances for getting pregnant quickly.

And keeping an accurate BBT chart calls for some serious commitment. Even if you’re diligent about taking your temperature as soon as you wake up every single morning, there are still other factors that can affect your body temperature and make it harder to pinpoint ovulation.

BBT charts are a safe, simple and reasonably accurate way to figure out when you’re ovulating. But you’ll need to be focused and patient to get the best results — and even then, they aren’t always reliable. If you’re looking for a quicker, more precise method for tracking your ovulation, you might want to think about using ovulation test strips instead.