BBT Calculator — Basal Body Temperature & Ovulation
Enter your follicular-phase baseline temperature and today's reading to check whether a thermal shift has occurred. The calculator applies the standard cover-line method and identifies whether your temperature suggests the follicular or luteal phase of your cycle.
Temperature unit
Significant thermal shift detected
- 1
Baseline BBT
36.3 °C - 2
Cover line (baseline + 0.1 °C)
36.4 °CCover line = baseline + 0.1 °C (≈ 0.18 °F). Three consecutive readings at or above this level confirm ovulation. - 3
Temperature rise above baseline
36.6 − 36.3 = 0.30 °C
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After ovulation, progesterone raises BBT by 0.2–0.5 °C (0.4–1.0 °F) above the follicular baseline. Cover line = highest of the 6 pre-shift readings + 0.1 °C. Three consecutive readings at or above the cover line confirm ovulation. Measure first thing every morning with a basal thermometer.
Formula
How this is calculated
Basal body temperature is the lowest body temperature reached during rest, measured immediately on waking before any activity. Progesterone, which rises sharply after ovulation, raises the BBT by 0.2–0.5 °C (0.4–1.0 °F) and sustains it throughout the luteal phase until menstruation. Charting this daily provides a retrospective confirmation of ovulation — you can confirm ovulation happened but typically only one to two days after the fact.
The cover-line method establishes a reference level as the highest of the six pre-shift temperatures plus 0.1 °C. A confirmed thermal shift requires three consecutive readings at or above the cover line — this rules out random variation. The calculator compares the current reading to the baseline to estimate whether a shift has occurred. A difference of ≥ 0.2 °C above baseline (and above the cover line) is consistent with post-ovulation.
For reliable BBT charting, measure at the same time every morning (within 30 minutes) using a dedicated basal thermometer accurate to 0.05 °C or 0.1 °F, before getting out of bed, after at least three hours of sleep. Fever, alcohol, late bedtime, travel, or illness can create false temperature spikes. This calculator provides guidance only — it does not replace a continuous chart tracked over multiple cycles, and is not a contraceptive method.
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The rise typically occurs the day after ovulation, triggered by progesterone. In a 28-day cycle, ovulation usually occurs around days 12–16, so the thermal shift appears around days 13–17. However, cycle length and ovulation timing vary considerably between women and even between cycles in the same woman.
The cover line is drawn one tenth of a degree above the highest temperature recorded in the six days before the thermal shift. Any reading at or above this line counts as an elevated reading. Three consecutive elevated readings confirm ovulation has occurred.
BBT alone is not recommended as a reliable contraceptive method because it confirms ovulation retrospectively — by the time you see the rise, the fertile window has largely passed. For fertility awareness-based contraception, BBT is combined with cervical mucus observation (the Symptothermal Method) and requires formal training.
Magħruf ukoll bħala
TG we-Calculate Editorial Team. (2026). BBT Calculator — Basal Body Temperature & Ovulation [Online calculator]. TG we-Calculate. https://we-calculate.com/mt/calculator/bbt-calculator
TG we-Calculate Editorial Team. "BBT Calculator — Basal Body Temperature & Ovulation." TG we-Calculate. 2026. https://we-calculate.com/mt/calculator/bbt-calculator.
TG we-Calculate Editorial Team, "BBT Calculator — Basal Body Temperature & Ovulation," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/mt/calculator/bbt-calculator
@misc{wecalculate_bbt_calculator, title = {BBT Calculator — Basal Body Temperature & Ovulation}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/mt/calculator/bbt-calculator}}, year = {2026}, note = {TG we-Calculate} }
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