Quick Answer — What Does a BBT Shift Actually Mean?
A basal body temperature (BBT) chart looks for one thing: a sustained rise of about 0.4°F (0.2°C) or more above your follicular-phase baseline, held for several consecutive days. That rise is caused by progesterone released after ovulation, and it typically confirms that ovulation has already happened — BBT is a retrospective confirmation method, not a forward-looking prediction of when ovulation will occur.
Before ovulation (the follicular phase), most people's waking temperature sits in a fairly flat range around 97.0-97.5°F. After ovulation (the luteal phase), progesterone pushes that baseline up to roughly 97.6-98.6°F, staying elevated until your next period, when it drops back down (or stays elevated if pregnancy occurs).
This calculator compares today's reading against your recent baseline average and, if a shift is confirmed, estimates which cycle day ovulation likely occurred on and projects your next period using your typical luteal phase length — the far more consistent half of the cycle.