BBT Fertility Calculator

Analyze your Basal Body Temperature (BBT) chart for ovulation confirmation.

Temperature Unit

Avg. Pre-Shift Temp Average of the last 6 non-elevated temps.

First High Temperature The first temperature above the cover line.

Number of High Temp Days The number of days temperature remained elevated.

Ovulation Analysis

Days Since Confirmed Shift

Ovulation Confirmation Status

BBT Interpretation Rules

Requires 3 consecutive days of higher temperature to confirm ovulation.

Basal Body Temperature (BBT) for Fertility Tracking

The **BBT Fertility Calculator** is a natural family planning tool that uses daily **Basal Body Temperature (BBT)** measurements to pinpoint the day of **ovulation** and identify the post-ovulatory phase. **BBT charting** relies on the slight ($0.5^\circ \text{F}$ to $1.0^\circ \text{F}$) rise in resting body temperature that occurs after **ovulation** due to the surge in the hormone progesterone.

The BBT Shift Rule

To confirm **ovulation** via **BBT charting**, the temperature must meet the following criteria (known as the shift rule):

The temperature must rise by at least $0.2^\circ \text{F}$ (or $0.1^\circ \text{C}$) above the "cover line" (the average of the past 6 pre-ovulatory temperatures).
The temperature must remain elevated for at least **3 consecutive days**.

If these criteria are met, **ovulation confirmation** is considered complete, and the fertile window is closed. This **BBT Calculator** checks if your recorded temperatures satisfy these rules.

BBT Charting and Natural Family Planning

**BBT charting** is a key component of the Sympto-Thermal Method of **natural family planning**. While it confirms **ovulation** *after* it has occurred, it helps predict future cycles and identifies patterns that can assist with **conception planning** or avoidance. Combining the **BBT** data with cervical mucus monitoring offers the highest accuracy in predicting the start of the next **fertile window**.

BBT Fertility FAQs

What is the "cover line" in BBT charting?

The "cover line" is the horizontal baseline temperature drawn on a **BBT chart**, representing the average of the lower pre-ovulatory temperatures. The temperature must rise significantly above this line for **3 consecutive days** to achieve **ovulation confirmation**.

How much does BBT temperature usually rise after ovulation?

The post-ovulation temperature shift, caused by progesterone, is typically subtle, ranging from $0.5^\circ \text{F}$ to $1.0^\circ \text{F}$ ($0.3^\circ \text{C}$ to $0.6^\circ \text{C}$) above the previous average. The calculator uses a minimum threshold of $0.2^\circ \text{F}$ ($0.1^\circ \text{C}$) for **ovulation confirmation**.

Can BBT charting predict the fertile window?

**BBT charting** primarily confirms **ovulation** retroactively. However, by tracking several cycles, a pattern emerges, allowing women to use this historical data to predict the likely start of the next **fertile window** for future **conception planning**.