OnlineCalculatorWhat BBT charting tracks
Basal body temperature is your temperature at complete rest, measured immediately on waking before any activity. It is tracked because a small sustained rise, typically around 0.2 to 0.5 °C, occurs after ovulation and persists through the remainder of the cycle.
The key word is sustained. A single elevated reading means little. Charting looks for a rise held across several consecutive days relative to the preceding baseline.
Why measurement discipline matters
- Measure at the same time each morning, before getting out of bed
- Use a thermometer with two-decimal resolution — ordinary fever thermometers are too coarse
- Use the same site and the same device throughout a cycle
- Note disruptions: poor sleep, illness, alcohol and travel all shift readings
BBT is retrospective, not predictiveThe temperature rise confirms that ovulation has already occurred. It does not forecast it. For anticipating the fertile window, BBT is usually combined with other indicators rather than used alone.
Interpreting a chart
Charts are interpreted as patterns across a full cycle, not as individual data points. If you are using BBT for family planning or because you have concerns about your cycle, discuss the chart with a healthcare professional who can interpret it alongside your wider history.