How to chart your basal body temperature (BBT)

November 11, 2026

Basal body temperature, or BBT, is your body's temperature at full rest, measured right after you wake up and before you do anything else. On its own, one reading tells you very little. Charted every morning for a whole cycle, the readings usually form a pattern: lower temperatures before ovulation and a small, lasting rise after it.

Why your temperature rises after ovulation

Once an egg is released, the leftover follicle becomes the corpus luteum, which makes progesterone. Progesterone acts on the part of the brain that sets body temperature and raises it slightly. Cleveland Clinic describes the usual rise as about 0.4 to 1 degree Fahrenheit (roughly 0.2 to 0.5 degrees Celsius).

The higher temperatures usually last until your next period. If you aren't pregnant, progesterone falls, your temperature drops back down, and bleeding typically starts within a day or two. If you are pregnant, progesterone stays up and so does your temperature. A run of high readings lasting well past your usual luteal phase can be an early hint of pregnancy, though a pregnancy test is the way to know.

How to take your BBT

The change is small, so the method matters more than the thermometer brand.

Use a basal thermometer that reads to at least one tenth of a degree. Regular fever thermometers often aren't precise enough.
Take your temperature as soon as you wake, before you sit up, talk, drink water or check your phone.
Try to take it at about the same time each morning, ideally after at least three hours of unbroken sleep.
Use the same spot every time. Most people use their mouth. Vaginal or rectal readings are steadier for some people, but don't switch partway through a cycle.
Write it down right away, starting on the first day of your period.

Make a note next to any reading taken on an unusual morning. Alcohol the night before, a fever or cold, poor or short sleep, stress, travel across time zones, some medications and breastfeeding can all throw off a reading. Marking these helps you set aside an odd number instead of mistaking it for a real shift.

The 3-over-6 rule and the coverline

Day-to-day readings bounce around a little, so you need a clear way to decide when the rise is real. A common method in fertility awareness teaching is the 3-over-6 rule.

Look for the first day your temperature is higher than each of the six days before it. Find the highest of those six earlier readings and draw a horizontal line just above it. That line is called the coverline. If you then get three temperatures in a row above the coverline, the shift is confirmed, and ovulation most likely happened around the day before the first high reading.

Some teaching programs add extra conditions, such as requiring the third high reading to sit a set amount above the coverline. Different methods have slightly different rules, so if you're relying on a chart for a decision, learn one method properly from a trained instructor or a detailed guide and stick with it.

What a BBT chart can tell you

A clear shift that holds is good evidence that you ovulated that cycle. Over several cycles, your charts can show roughly when you tend to ovulate and how long your luteal phase usually lasts. That can be useful information to bring to a doctor if you're trying to conceive or your cycles feel off.

Charting also helps you understand your own timing. Many people find their period stops being a surprise once they can see the temperature drop a day or two ahead of it.

What it can't tell you

BBT confirms ovulation after the fact. It does not predict it. By the time your temperature has risen, the egg has usually been released, and the most fertile days are the ones just before that. So a single cycle's chart won't warn you that ovulation is coming.

It also has other limits. Some people ovulate without a clear temperature shift, and a few have a rise that's hard to read. Irregular cycles, shift work and frequent illness make charts harder to interpret. A chart can't tell you about egg quality, if your tubes are open or if a pregnancy will continue.

If you're trying to avoid pregnancy, temperature alone is not a strong method. Fertility awareness methods that combine temperature with cervical fluid and a calendar can be quite effective with careful, consistent use. The NHS gives figures of up to 99% effective with perfect use, falling to about 76% when the instructions aren't followed exactly, which means around 24 in 100 people get pregnant in a year. These methods take a few cycles to learn and don't protect against STIs.

If you'd like to start, Barkat's cycle tab lets you log your morning temperature and keep the chart private on your phone.

This article is general information and not medical advice.

Sources

This is general information, not medical advice. If something about your cycle worries you, talk to a doctor or midwife.

Open Barkat

Next: Cervical fluid: what changes across your cycle and why