Pregnancy & Fertility
BBT Fertility Calculator
Progesterone heats you up, about 0.3–0.6 °C after ovulation. Enter your morning temperatures and detect the shift.
Basal body temperature (BBT) rises about 0.3–0.6 °C after ovulation due to progesterone. Tracking daily morning temperatures for a cycle shows a biphasic pattern — the shift confirms ovulation occurred and can guide timing. Dates are estimates — confirm timing with your provider and ultrasound where relevant.
Shift: +0.3–0.6 °C after ovulation.
Take temp before getting out of bed.
3+ raised days confirm ovulation.
Calculate Your BBT Fertility
Temperature Shift
What Is BBT Fertility?
Basal body temperature (BBT) is your temperature at full rest, taken immediately on waking, before moving. Progesterone released after ovulation raises BBT by 0.3–0.6 °C.
Charting BBT across a cycle reveals the biphasic pattern, a flat low phase, then a sustained rise, confirming that ovulation occurred. It confirms after the fact; ovulation tests predict beforehand.
How It Is Calculated
Ovulation confirmed when temp stays ≥ 0.3 °C above coverline for 3+ days
Example: Example: first-6 average 36.50°, last-3 average 36.97° → +0.47° shift detected
Reading the chart
| Pattern | Meaning |
|---|---|
| Sustained rise ≥ 0.3 °C, 3+ days | Ovulation occurred |
| Flat line all cycle | Possible anovulation |
| Erratic swings | Inconsistent measurement |
| Rise then early drop | Short luteal phase (discuss with clinician) |
Limitations
- BBT confirms ovulation retrospectively, use OPKs to time intercourse prospectively.
- Measure at the same time daily, before getting up, after 3+ hours of sleep.
- Illness, alcohol, and poor sleep distort readings.
Sources & Review
References used for this calculator’s formulas and thresholds:
BBT Fertility FAQ
How much does BBT rise after ovulation?
About 0.3–0.6 °C (0.5–1.0 °F), sustained for the rest of the cycle until the next period.
When should I take my temperature?
Immediately on waking, before sitting up or drinking, same time daily, after at least 3 hours of sleep.
Can BBT predict ovulation?
No, it confirms ovulation after it happens. Pair it with LH test strips to time intercourse in advance.
What if I never see a shift?
Anovulatory cycles happen occasionally. If several cycles show no rise, discuss with a clinician.
A note before you begin: this tool offers educational estimates only. It cannot replace advice from your doctor or another qualified professional.