BBT Chart (Basal Body Temperature) — Ovulation Estimator

Enter your daily basal body temperature to draw an automatic line chart and detect the shift from the low phase to the high phase (coverline method), showing your estimated ovulation date. Everything runs in your browser, and nothing is saved on a server.

How the Coverline Method (3-Over-6 Rule) Works

Step	What it involves
Record your low-phase temperatures	Before ovulation, the low phase tends to stay fairly stable and low since it is not yet affected by progesterone. At least 6 days of readings are needed as the basis for the calculation
Draw the coverline	A baseline (the coverline) is drawn just above the highest temperature from your most recent 6 low-phase days, adding a threshold of about 0.1°C (or 0.2°F)
Check for 3 consecutive days above the line	The shift to the high phase is confirmed once your temperature stays above the coverline for 3 days in a row. A rise lasting only 1-2 days is not counted
Work backward to estimate ovulation	Because the temperature rise is thought to be triggered by progesterone released after ovulation, the day before the first confirmed high-phase reading is used as the estimated ovulation date

The estimated ovulation date shown by this tool is only a statistical approximation based on basal body temperature patterns. It does not guarantee or diagnose your actual ovulation date or chance of pregnancy. For decisions about trying to conceive or contraception, please also consider ovulation predictor kits or a visit to a gynecologist.

What Is a BBT Chart?

A basal body temperature (BBT) chart plots the temperature you measure each morning at rest onto a line graph, using the two-phase (low and high) pattern to estimate the timing of ovulation. A woman's body temperature is said to rise by roughly 0.3-0.5°C after ovulation due to progesterone, the hormone released at that time, and this tool uses that temperature shift to give you a general idea of when ovulation occurred.

This tool uses the coverline method (also known as the 3-over-6 rule), a widely used approach in gynecology and Natural Family Planning (NFP). It draws a baseline (the coverline) by adding a threshold to the highest temperature from your most recent 6 low-phase days, then flags the shift to the high phase once your temperature stays above that line for 3 consecutive days. All temperatures you enter are processed only in your browser and are never sent to or stored on a server.

How to Use

Choose your temperature unit Select Celsius (°C) or Fahrenheit (°F) to match the thermometer you normally use
Enter your daily basal body temperature Log the temperature you measure right after waking, before moving around, together with the date for each day (the chart appears once you have at least 3 days of data)
Add more rows As your log grows, use "+ Add row" to create more input fields (up to 60 days)
Check the chart and result View your temperature trend on the chart, along with the coverline and estimated ovulation date once a shift to the high phase has been confirmed

Tips for getting more out of it

Taking your temperature at the same time and under the same conditions every morning (right after waking, before moving) improves the accuracy of the chart.
On days affected by illness, alcohol, lack of sleep, or a different measurement time, your temperature may not reflect your usual pattern, so it helps to leave a note you can refer back to later.
A single cycle is sometimes not enough to see a clear two-phase pattern. If possible, we recommend recording 2-3 cycles and comparing the trend across them.
If your high phase repeatedly ends in under 10 days, or the two-phase pattern stays unclear, it is worth considering a visit to a gynecologist.

Use Cases

Trying to conceive with timed intercourse

Keep an ongoing record of your temperature pattern as a guide for timing intercourse around your estimated ovulation date

A supplementary reference for contraception

Tracking when the shift to the high phase is confirmed can help you understand your cycle and general health trends (this is not a substitute for a reliable method of contraception)

Bringing records to a gynecology appointment

A chart covering several cycles can serve as material to help explain your cycle patterns to a doctor during a gynecology visit

Noticing irregular cycles early

A weak two-phase pattern or a shorter-than-usual high phase can be an early clue that something is different from your usual cycle

Glossary

Basal body temperature: The temperature measured at rest in the morning before getting up and moving around. It is typically taken with a basal thermometer right after waking
Low phase: The period from the start of menstruation until ovulation, during which temperature stays relatively low
High phase: The period from ovulation until the start of the next period, during which progesterone keeps the temperature slightly elevated
Coverline: A baseline drawn by adding a threshold to the highest temperature of your most recent 6 low-phase days. Staying above this line for 3 consecutive days is treated as the shift to the high phase
Ovulation date: The day an egg is released from the ovary. On a BBT chart, the day before the first confirmed high-phase reading is generally used as the estimate
Biphasic pattern: A pattern in which a BBT chart splits clearly into two distinct stages, the low phase and the high phase. It is considered one indicator that ovulation has occurred

Frequently Asked Questions

The standard approach is to measure it right after waking, at rest, before you get up and move around. Keeping a basal thermometer (one that measures to two decimal places) within reach of your bed and taking your temperature the moment you wake makes it easier to keep the conditions consistent.

Detecting the shift requires 6 days of low-phase readings to calculate the coverline, plus 3 consecutive days above that line. If you have too few readings, your temperature swings are small, or you have not ovulated yet, the shift may not be confirmed. Continuing to log your temperature may resolve this over time.

Choose whichever unit matches your thermometer. Switching units automatically adjusts the threshold and valid range to match, but note that any temperatures you have already entered will be cleared and need to be re-entered.

No, it is not saved. Everything you enter is calculated entirely within your own browser, is never sent to a server, and will be cleared if you reload the page.

No, it cannot. A high phase that lasts longer than usual can suggest a possibility of pregnancy, but this is only a statistical indicator. Confirming pregnancy requires a pregnancy test and a visit to a gynecologist.

Side Note — How the Idea of Basal Body Temperature Spread

Using basal body temperature to track ovulation timing is said to have spread after a Dutch gynecologist reported the link between progesterone and body temperature in the early 20th century. Since then, as basal thermometers became more widely available, the method spread around the world as an easy way for people to track their cycles at home.

The coverline method (the 3-over-6 rule) is one of the ideas that developed within Natural Family Planning (NFP), an approach that uses physical signs to judge more and less fertile times of the cycle. By basing the threshold on the highest temperature of the low phase, it became established as a practical rule that is less thrown off by small day-to-day temperature fluctuations.

Unlike an ordinary thermometer, a basal thermometer measures to two decimal places (increments of 0.01 degrees). This is because the basal body temperature shift is only about 0.3-0.5 degrees, so capturing that small difference accurately matters. As digital thermometers have become more precise, measuring and recording basal body temperature at home has become easier than ever before.

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