Basal Body Temperature Shift Calculator

About this calculator

Basal body temperature (BBT) — your resting body temperature measured immediately upon waking, before any activity — typically rises by about 0.2-0.5°C (0.4-1.0°F) after ovulation due to the thermogenic effect of progesterone, and stays elevated until the next period. This calculator compares your average pre-ovulation temperatures against a post-ovulation reading to detect whether that sustained thermal shift has occurred, which is the classic sign that ovulation has already taken place.

BBT charting is a long-standing method used in fertility awareness and natural family planning, letting people trying to conceive (or avoid pregnancy) identify their fertile window retrospectively by tracking this temperature pattern across a cycle, often alongside cervical mucus observations. Because the shift only confirms ovulation after the fact rather than predicting it in advance, it's typically combined with other fertility signs for cycle tracking.