What is the coverline in basal body temperature (BBT) charting?

By Li

The key of BBT charting is to identify the date when the bbt thermal shift (BBT spike) happens. The coverline is a horizontal line drawn in your BBT chart after BBT spike pattern is established. It is drawn just above the last temperature prior to the first rise in temperature. Coverline is a visual tool to help you identify the bbt thermal shift and show the biphasic BBT pattern so that you may observe the trend rather than individual temperatures.


A typical bbt spike will likely be between 0.4 °F and a full degree (or even a bit more). The minimum basal temperature increase could be 0.2°F. To draw a coverline on your BBT chart, you should follow the following steps

  1. You identify three continuous daily basal temperatures that are at least 0.2 or two-tenths of a degree higher than the previous six days. (6 Low and 3 high principle )
  2. Highlight those previous six days
  3. Find the highest of these six highlighted temperatures
  4. Draw a straight line that is 0.1 or one-tenth of a degree higher than that of the highest of the highlighted temperatures
  5. This line is your coverline
  6. The coverline should run straight horizontally across your chart

Here is an example of coverline on BBT charting. The first identified temperature above coverline is your BBT spike.

While the coverline is pretty useful when you use bbt charting to track your ovulation cycle, it is a time-consuming task to track and draw every cycle. Good news is that some of smart APPs like Premom integrate with the smart basal thermometer and support drawing the coverline automatically for you so that you are able to see the biphasic BBT pattern easily.

Amanda St. Aubin