Understanding the BBT Coverline
Basal body temperature (BBT) is your body's lowest resting temperature, measured first thing in the morning before you get up, eat, drink, or move around. Charted across a menstrual cycle, BBT shows a small but reliable two-phase pattern: lower readings before ovulation and higher readings after it, driven by the hormone progesterone. The coverline is the horizontal line that separates those two phases, and this calculator draws it for you.
The coverline formula
The coverline is set one tenth of a degree Fahrenheit above the highest of the six low temperatures recorded on the days just before your temperature rises:
- Coverline = highest of the 6 pre-ovulatory temperatures + 0.1°F.
- The six temperatures are the cluster of low readings immediately before the first day your waking temperature jumps at least 0.2°F.
- Ovulation is confirmed retrospectively by the 3-over-6 rule: once three consecutive temperatures land above the coverline, ovulation is presumed to have happened on the day before the first high reading.
How to measure BBT accurately
- Take your temperature at the same time every morning, after at least three hours of continuous sleep, before you sit up.
- Use a dedicated basal thermometer that reads to two decimal places (0.01°F); ordinary fever thermometers are too coarse to catch a shift of a few tenths of a degree.
- Record the reading immediately. Illness, alcohol, a late night, or a different measurement time can raise a single reading and produce a false spike.
Reading your chart
A clear thermal shift is a run of temperatures that stays above the coverline until your next period. If temperatures cross the line for a single day and drop back, ovulation is not yet confirmed — wait for three sustained highs. Because the rise appears only after ovulation, BBT confirms ovulation after the fact rather than predicting it in advance.
When to consult a professional
BBT charting is an estimation tool for understanding your cycle, not a diagnosis or a stand-alone method of contraception. If your chart is consistently flat, erratic, or you are tracking to conceive or to avoid pregnancy, discuss it with a licensed healthcare provider or a certified fertility-awareness instructor.