What is basal body temperature (BBT)?

The basal temperature is the body's temperature at complete rest. It is measured immediately after waking up and before any activity begins, using a thermometer that displays values to two decimal places. Since it is a reference value in a state of rest, even slight hormonal changes are clearly reflected in daily measurements.

How does basal body temperature change before and after ovulation?

The basal temperature is usually low before ovulation. After ovulation the progesterone level rises, which causes a rise of about 0.3 °C (0.5 °F) in the basal temperature and keeps it at this higher level during the luteal phase. This sustained increase can be seen on the chart. It is not used to predict ovulation but to confirm it afterwards.

How can basal body temperature (BBT) be corrected if it is measured at different times?

The longer the rest period is, the higher the body temperature will be. So the value (normal time - actual time) * rate is added to the readings. The normal rate is 0.05 °C per 30 minutes (or 0.1 °C per hour). If the reading is later than usual, then the values are corrected downwards and if it's earlier they're corrected upwards. This tool does the calculations for you and allows you to choose between Celsius or Fahrenheit.

What factors can cause BBT readings to be unreliable?

Short or interrupted sleep, alcohol the night before, fever or other illness and traveling across time zones can cause a morning reading to be skewed. Mark that date accordingly and check not just individual temperature readings but also the overall trend. There is no reliable way to correct for fevers so it's better to mark those data points than trust them.

Can BBT be used to detect ovulation or pregnancy?

A temperature rise that lasts three days or longer can be an indication that ovulation has already occurred in this cycle. If the temperature stays high when it should normally drop, this could be an early sign of pregnancy. However, neither factor is a basis for diagnosis. Basal body temperature is not a contraceptive method, so important situations should be discussed with a doctor and confirmed by a pregnancy test.

BBT Calculator (Basal Body Temperature)

Your reading

Measured temperature

Usual measurement time

Time you measured today

Adjustment rule

Both heuristics are conservative; the Celsius rule is Omni's, the Fahrenheit rule is common in US fertility-awareness guides.

Celsius rule — 0.05 °C per 30 min

Fahrenheit rule — 0.1 °F per 30 min

Adjusted temperature

Minutes off your usual time

Optional checks

Check this reading's reliability

Flag anything that can distort BBT. Flags never change the number — they tell you how much to trust it.

Compare with your coverline

Enter the highest of your six temperatures before the rise to draw a coverline.

Adjusted temperature

°C

You measured at your usual time, so no timing adjustment is needed.

Understanding your BBT chart

Measure consistently: same time every morning, before you sit up, eat, drink, or talk. Use a thermometer with two decimal places, after at least three hours of solid sleep.

Read the trend, not one day: BBT rises after ovulation, so it confirms rather than predicts it. Pair it with cervical-mucus or ovulation-test signs for an advance signal.

Not birth control, not medical advice. This tool supports charting and education only. Talk with a clinician about fertility, contraception, or a possible pregnancy.

The basal temperature rises gradually the longer you rest. Therefore, readings taken at a different time than your usual reading time can be higher or lower for no apparent reason. This tool corrects the actual measured basal temperature to the value of the normal reading time and prevents unusual data on a particular morning from distorting the graph.

Please enter the measured values, usual measurement time and today's measurement time. The corrected body temperature will be displayed on the right. With the optional functions you can mark affected measurements or compare a value with a comparison line.

What is base temperature?

The basal temperature is the body temperature when the body is in a completely resting state. Measure it immediately after waking up and before starting any activities. Use a highly sensitive thermometer that can display values with two decimal places for measurement.

The basal temperature is important in determining the fertile window because progesterone has a warming effect. After ovulation, progesterone rises and your basal temperature will increase by about 0.3 °C (0.5 °F). By recording this change over a cycle you can identify the relatively low-temperature phase of your cycle (the follicular phase) and the higher-temperature phase (luteal phase).

Why do values change depending on when they are measured?

The longer you stay in bed the higher your body temperature will be. So if you take your temperature later than usual it will appear to be higher and if you take it earlier it will appear lower. The correction assumes that your body temperature changes at a constant rate, and adjusts the values back to a reference time.

\text{Adjusted} = \text{Measured} + (\text{Usual time} - \text{Actual time}) \times \text{rate}

There are two commonly used conservative speeds to choose between in this calculator.

\text{Celsius rule: } 0.05\,^\circ\text{C per 30 min} = 0.1\,^\circ\text{C per hour}

\text{Fahrenheit rule: } 0.1\,^\circ\text{F per 30 min} = 0.2\,^\circ\text{F per hour}

Example calculation:

Normally the basal temperature is measured at 6:30 am but today you overslept and took it at 7:00 am when it was 36.60 °C. As the measurement was taken 30 minutes late, according to the Celsius rule, 0.05 °C will be deducted from the value.

36.60 + (6.5 - 7.0)\times 0.1 = 36.60 - 0.05 = 36.55\,^\circ\text{C}

Due to this half-hour delay, the original value was about 0.05°C higher. After correction, the value is returned to its correct position around reference time 6:30.

Overview of measuring times and correction values.

In the case of the Celsius rule (each 0.1 °C), the size of the correction is determined solely by how much the time of measurement deviates from the usual time of measurement.

You measured	Adjustment to the reading
1 hour earlier	+0.10 °C (+0.18 °F)
30 minutes earlier	+0.05 °C (+0.09 °F)
At your usual time	no change
30 minutes later	−0.05 °C (−0.09 °F)
1 hour later	−0.10 °C (−0.18 °F)

Reading the chart: Temperature changes and reference line

The reference line is a horizontal line drawn above the highest value of the first six measurements before the temperature rise begins. When a measurement crosses this line three times in succession, it indicates that there has been a change in body temperature and it is confirmed that ovulation has already occurred.

If the reference line function is enabled and the highest value of the first six readings before the temperature change begins is entered, then the calculation tool will draw a reference line just above it. For Celsius this is 0.05 °C and for Fahrenheit 0.1 °F. It also indicates whether the current correction value is above or below the reference line.

Factors that can affect the reliability of measurements

Basal temperature can be slightly influenced. Factors such as too little or irregular sleep, alcohol consumption the night before, fever or illness and travelling across time zones can cause a reading to be higher or lower on a particular morning. The reliability function allows these situations to be marked. Instead of making arbitrary numerical corrections, this function shows how reliable individual data points are. This allows values to be marked or discarded without distorting the trend.

This calculator is for educational purposes only and to help you create charts. It does not provide contraception or medical advice. If you would like information about your fertility, contraception or the likelihood of getting pregnant please consult a doctor.

Frequently asked questions

What is basal body temperature (BBT)?: The basal temperature is the body's temperature at complete rest. It is measured immediately after waking up and before any activity begins, using a thermometer that displays values to two decimal places. Since it is a reference value in a state of rest, even slight hormonal changes are clearly reflected in daily measurements.
How does basal body temperature change before and after ovulation?: The basal temperature is usually low before ovulation. After ovulation the progesterone level rises, which causes a rise of about 0.3 °C (0.5 °F) in the basal temperature and keeps it at this higher level during the luteal phase. This sustained increase can be seen on the chart. It is not used to predict ovulation but to confirm it afterwards.
How can basal body temperature (BBT) be corrected if it is measured at different times?: The longer the rest period is, the higher the body temperature will be. So the value (normal time - actual time) * rate is added to the readings. The normal rate is 0.05 °C per 30 minutes (or 0.1 °C per hour). If the reading is later than usual, then the values are corrected downwards and if it's earlier they're corrected upwards. This tool does the calculations for you and allows you to choose between Celsius or Fahrenheit.
What factors can cause BBT readings to be unreliable?: Short or interrupted sleep, alcohol the night before, fever or other illness and traveling across time zones can cause a morning reading to be skewed. Mark that date accordingly and check not just individual temperature readings but also the overall trend. There is no reliable way to correct for fevers so it's better to mark those data points than trust them.
Can BBT be used to detect ovulation or pregnancy?: A temperature rise that lasts three days or longer can be an indication that ovulation has already occurred in this cycle. If the temperature stays high when it should normally drop, this could be an early sign of pregnancy. However, neither factor is a basis for diagnosis. Basal body temperature is not a contraceptive method, so important situations should be discussed with a doctor and confirmed by a pregnancy test.

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Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.

References

MedlinePlus (NIH) — Ovulation and basal body temperature
U.S. National Library of Medicine patient guidance on tracking ovulation, including the basal body temperature method.
ACOG — Fertility Awareness-Based Methods of Family Planning
American College of Obstetricians and Gynecologists FAQ on fertility awareness, including basal body temperature and its limits.
Basal body temperature — Wikipedia
Encyclopedic overview of BBT, the biphasic pattern, the coverline, and charting methods.