What’s the Best Thermometer for Measuring Basal Body Temperature?

wrote this on November 25, 2014

The world of basal thermometers can feel pretty endless when you’re trying to figure out which is right for you, especially if you’re still new to the whole fertility charting thing.

Even if you’re a seasoned user of the sympto-thermal method of fertility awareness, navigating the world of basal thermometers can feel daunting. So, to make your search a bit less stressful, I’ve compiled a few things to keep in mind while searching for the perfect fit.

Glass or Digital?

Ah, the age old question: should I use a glass or digital basal thermometer?

Even though glass thermometers are pretty old school, their simplicity and reliability are hard to beat. However, there are definitely some things to keep in mind before taking the plunge. The big plus with a glass thermometer is that it will last pretty much indefinitely (unless you break it, of course). A broken glass thermometer can be a hazard if it’s filled with mercury; luckily, most that are sold today are mercury-free.

One of the cons of glass thermometers is that they can take up to 5 minutes to get an accurate temperature reading, which can feel like an eternity when you’re still half-asleep. And, unlike their digital brethren, glass thermometers don’t beep when they’re finished measuring your temp, so it will be up to your discretion to figure out when it’s finished. They also need to be shaken down after each use.

There are a few cons of using a digital basal thermometer, too: some of them beep the whole time your temperature is being measured—which may not be appreciated by a snoozing partner—and their batteries need changing every so often. I’ll be the first to admit that I get peeved when I wake up to take my temperature, only to find that the battery is dead. On the plus side, many digital basal thermometers have a back-lit display, which makes it easy to read in the dark. They also tend to be a bit faster, with some getting a measurement in about 60 seconds.

Thermometer Accuracy

Some basal thermometers read to the nearest 1/10th °F (97.6 °F, for example) while others measure to the nearest 1/100th °F (like 97.68 °F). So…which should you use?

There’s no hard-and-fast rule about your thermometer’s accuracy, except if you chart in Celsius. If you chart in Celsius, it’s important that your thermometer measures to the 1/100th degree (36.38 °C, for example) because of the differences in the two temperature scales.

If you’re a Fahrenheit charter, I recommend charting to the 1/10th degree for the sake of simplicity. The extra digit really isn’t necessary to spot your temperature shift, and can actually make things more confusing. Spotting your temperature shift shouldn’t feel like a math test!

If your thermometer happens to measure in increments of 0.01 °F but you don’t want to waste your time with unnecessary digits, simply drop the last digit from your temperature reading (97.76 °F becomes 97.7 °F, for example). You can round if you prefer—97.76 °F rounds up to 97.8 °F and 97.74 °F rounds down to 97.7 °F—but it isn’t necessary.

Can I use a fever thermometer?

What’s the difference between a basal thermometer and a regular old fever thermometer, anyway? Not a whole lot, as it turns out. Many fever thermometers are accurate to the 1/10th degree Fahrenheit, making them perfect for measuring your basal body temperature.

However, there are some fever thermometers that measure in increments of 0.2 °F (97.6, 97.8, 98.0, etc), which just won’t cut it. If you have a fever thermometer laying around at home, make sure it measures in increments of 0.1 °F before using it to measure your basal body temperature.

Check out Groove’s store to see what basal thermometer we recommend.

Jennifer Aldoretta is the cofounder and CEO of Groove. She is an entrepreneur, engineer, and biohacker who is obsessed with periods, nutrition, hormones, and the microbiome.