How to read a BBT chart
BBT is most useful for confirming ovulation after it happens, not predicting it in advance. Look for a sustained temperature rise, then combine the pattern with cervical mucus or ovulation tests if timing matters.
| Before ovulation | Temperatures are usually lower and can wiggle day to day. |
|---|---|
| After ovulation | A rise that stays elevated for at least three days suggests ovulation already happened. |
Sleep, illness, alcohol, travel, stress, and inconsistent timing can make a chart noisy.
BBT chart example
A typical basal body temperature chart is biphasic: lower temperatures before ovulation, then a sustained rise after ovulation. The exact numbers matter less than the pattern.
| Cycle moment | Example pattern | How to read it |
|---|---|---|
| Before ovulation | Lower, slightly wavy readings | Estrogen-dominant days often sit in your lower personal range. |
| Ovulation shift | One small dip or a first higher reading | The shift is only meaningful if higher temperatures continue. |
| After ovulation | At least 3 higher readings in a row | A sustained rise usually confirms ovulation already happened. |
| Disruptions | Single-day spikes or drops | Illness, poor sleep, alcohol, travel, and late measuring can distort one reading. |
Basal body temperature (BBT) is your body's lowest resting temperature, measured immediately upon waking before you move around. It is one of the most reliable ways to confirm that ovulation has occurred, and over several cycles, it can help you predict when ovulation typically happens for you.
How BBT Relates to Ovulation
After ovulation, your body releases progesterone, which causes your basal temperature to rise by about 0.2-0.5 degrees Celsius (0.4-1.0 degrees Fahrenheit). This shift persists for the rest of your luteal phase. If you are pregnant, it stays elevated. If you are not, it drops as progesterone falls and your period begins.
This temperature rise confirms that ovulation happened. It does not predict it in advance.
How to Measure Your BBT
- Take your temperature at the same time every morning, ideally after at least 3 hours of uninterrupted sleep.
- Measure before getting out of bed or talking, moving around, or drinking anything.
- Use a basal thermometer that reads to two decimal places.
- Log the reading immediately (keep your thermometer and a notebook or phone within reach).
Reading Your BBT Chart
Look for a sustained temperature shift: 3 or more days where your temperature is higher than the previous 6 days. This is the coverline method. Once you see this shift, ovulation has occurred. Over several cycles, you will start to see your typical pattern.
Temperatures before ovulation are typically lower (your follicular phase). After ovulation, they are consistently higher (your luteal phase). The shift between these phases marks ovulation.
What Can Disrupt Your Readings
BBT is sensitive to external factors. Common disruptors include:
- Waking at a different time than usual
- Illness or fever
- Alcohol the night before
- Poor sleep or waking during the night
- Mouth breathing while sleeping
Note these disruptions in your chart so you can discount those readings.
Combining BBT with Other Methods
BBT confirms ovulation but does not predict it. For the most complete picture, combine BBT with cervical mucus monitoring (which peaks before ovulation) and ovulation predictor kits (OPKs, which detect the LH surge). Use our fertility calendar to see your full cycle pattern.
Start Tracking Your Cycle
Bloom helps you log symptoms and temperatures to understand your unique cycle patterns.
Frequently Asked Questions
Can BBT predict ovulation in advance?
No. BBT only confirms ovulation after it has happened, because the temperature rise is caused by progesterone released after the egg is released. Use cervical mucus monitoring or ovulation predictor kits for advance prediction.
What if my BBT temperatures are erratic?
Erratic readings are common when wake times vary, if you have been drinking alcohol, are ill, or if you breathe through your mouth. Try to measure at the same time every day after at least 3 hours of sleep. Give it 2-3 cycles to see patterns emerge.
Do I need a special thermometer for BBT?
Yes, a basal thermometer that reads to two decimal places (e.g. 36.54C) is recommended. Standard fever thermometers only read to one decimal place and are not sensitive enough to catch the small shifts involved in ovulation tracking.