BBT Calculator - Basal Body Temperature

Basal body temperature, or BBT, is the body's resting temperature measured immediately after waking and before getting out of bed. Progesterone released after ovulation generally causes a small sustained rise, often around 0.2 to 0.5 degrees Celsius or 0.4 to 1.0 degrees Fahrenheit. Charting daily readings can help a person recognize a biphasic pattern: lower temperatures before ovulation and higher temperatures afterward. The pattern confirms a shift retrospectively rather than reliably predicting it from one reading. For consistent measurements, use a basal thermometer with sufficient precision and take the reading at approximately the same time each morning after at least several hours of uninterrupted sleep. Use the same measurement route according to the thermometer instructions. Record the value before moving around, eating, drinking, or checking other morning tasks. Illness, fever, alcohol, disrupted sleep, travel, shift work, stress, medication, room changes, and a different measurement time can all disturb a reading and should be noted on the chart. This calculator converts the entered temperature between Celsius and Fahrenheit. It also provides a calendar estimate based on the common assumption that ovulation occurs about 14 days before the next menstrual period. The estimated fertile window begins five days before that day and ends one day afterward, reflecting sperm survival and the shorter life of the egg. The arithmetic is transparent, but real luteal-phase length and ovulation timing vary among people and from cycle to cycle, especially when cycles are irregular. A single high temperature does not confirm ovulation. Fertility-awareness methods generally look for at least three higher readings compared with an earlier baseline and may combine temperature with cervical mucus or other observations under a defined method. Because BBT rises after ovulation, the most fertile days may have already passed when the shift becomes clear. Anyone using fertility awareness to avoid pregnancy should learn and follow a validated method rather than relying on this calculator's calendar phase. BBT tracking can support conversations about cycle patterns, but it cannot diagnose infertility, pregnancy, thyroid disease, infection, or a hormonal condition. Persistent fever requires appropriate medical attention, and prolonged or very irregular cycles may warrant professional advice. If trying to conceive, record complete daily charts over several cycles and share concerns with a qualified clinician. Use the estimated ovulation day and fertile window as planning context only, while treating the observed multi-day temperature pattern as more informative than a calendar projection.