What this calculator does
BBT stands for basal body temperature, the temperature your body sits at first thing in the morning, before getting up, eating or moving around. Charting it daily is one of the oldest fertility-tracking methods, because progesterone released after ovulation nudges that resting temperature up by a small but real amount, usually somewhere between 0.3°C and 0.5°C, and that rise tends to hold for the rest of the cycle.
This is a different method from predicting fertile days off cycle length alone. Cycle-length prediction looks backward at past cycles and estimates forward; BBT charting looks at what your body actually did this cycle, by finding the point where the daily readings step up and stay up. Paste in a run of readings and this calculator looks for that sustained shift.
The formula
The calculator uses a version of the 3-over-6 rule that BBT charts have long relied on: for each day, it takes the highest reading from the previous 6 days as a rough baseline, and looks for the first point where 3 readings in a row all sit above that baseline, with at least one of the three at least 0.2°C higher. That combination of "3 higher, sustained" is what separates a real shift from ordinary day-to-day wobble in body temperature.
| Term | Meaning |
|---|---|
| BBT | Basal body temperature: resting temperature taken immediately on waking, before any activity. |
| Coverline | A reference line drawn near the top of the pre-ovulation readings, used to judge whether later readings have shifted above it. |
| 3-over-6 rule | A shift is confirmed once 3 consecutive readings all exceed the highest of the previous 6, with the rise reaching at least 0.2°C. |
| Luteal phase | The second half of the cycle, after ovulation, when the elevated temperature is expected to hold. |
The inputs explained
| Field | What to enter |
|---|---|
| Daily BBT readings, in order (°C, comma separated) | Paste in one reading per day, in order, separated by commas. Use the same unit (Celsius) throughout, and ideally the same thermometer taken at the same time each morning. |
When to use it
Confirming ovulation happened, after the fact
BBT charting cannot predict ovulation in advance; it confirms it happened once the rise appears, which is why it is typically used alongside, not instead of, other signs earlier in the cycle.
Checking whether a cycle was likely ovulatory
A cycle with no clear sustained rise is worth discussing with a doctor if it happens repeatedly, since it may point to an anovulatory cycle rather than simply noisy data.
Cross-checking against other fertility signs
Cervical mucus changes or ovulation predictor kits often flag the fertile window a day or two before the temperature rise appears, so BBT charting works best as a confirmation alongside those, not the only method used.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the detected shift changes across different reading patterns
The same detection rule applied to four different runs of daily readings, showing how the day of the shift and its size vary.
| Reading pattern | Likely ovulation day | Sustained rise begins | Size of the rise |
|---|---|---|---|
| Rise on day 7 | Day 6 | Day 7 | 0.40°C |
| Rise on day 9 | Day 8 | Day 9 | 0.30°C |
| Rise on day 8 | Day 7 | Day 8 | 0.50°C |
| Rise on day 11 | Day 10 | Day 11 | 0.35°C |
Questions
How many days of readings do I need?
At least 10, and ideally readings that span from early in the cycle through a week or so past the expected rise. Fewer than that and there is not enough pre-rise baseline for the 3-over-6 comparison to mean much.
What if no rise is detected?
It can mean ovulation has not happened yet in the data supplied, the readings do not yet extend far enough past it, or the rise in this particular cycle is smaller or noisier than this simple rule can pick up. One flat cycle is not unusual; a repeated pattern of no detected rise is worth raising with a doctor.
Does illness or poor sleep affect the readings?
Yes. Fever, alcohol, disrupted sleep, or taking the temperature at a different time than usual can all shift a single reading enough to confuse the pattern. Charting works best with readings taken under consistent conditions, and an odd single day is usually just noise.
Is this a reliable way to avoid or plan pregnancy on its own?
BBT charting only confirms ovulation after it has already happened, so it identifies the fertile window too late to be used alone for planning. It is best treated as one input alongside other fertility signs, and anyone relying on cycle tracking for family planning should talk it through with a doctor.
For fertile-window prediction based on cycle length instead of daily temperature, see the ovulation calculator.