BBT chart & ovulation detector

After ovulation, the empty follicle produces progesterone, and progesterone raises your resting body temperature by roughly half a degree to a degree Fahrenheit. Charting your waking temperature every morning makes that step visible. The classic fertility-awareness rule — sometimes called three-over-six — calls a thermal shift when three consecutive reliable temperatures all sit at least 0.4°F (about 0.2°C) above the highest of the previous six. Ovulation is then estimated as the day before the first elevated reading. It's a retrospective confirmation that ovulation happened and roughly when, which is genuinely useful for understanding your cycle — but it looks backward, never forward.

No — and this is the most misunderstood thing about temperature charting. The rise happens after ovulation, once progesterone kicks in, so by the time your chart shows a shift, the egg has already been released and your most fertile days are behind you. Sperm live up to about five days, so the fertile window opens well before the temperature moves. If your goal is timing intercourse ahead of ovulation, use a forward-looking method — cycle-based estimates like our ovulation calculator, or ovulation predictor kits that detect the LH surge a day or so before ovulation. Use the BBT chart afterward to confirm the prediction was right.

Take it immediately when you wake — before you sit up, get out of bed, talk, or drink anything — after at least three hours of unbroken sleep, at about the same time each morning. Use a basal thermometer, which reads to 0.1°F (0.05°C); an ordinary fever thermometer is too coarse to catch a half-degree shift. Keep the site consistent too (oral is most common), because oral, vaginal, and ear readings differ. The routine matters more than perfection: a temp taken an hour late or after getting up isn't ruined data, it's just less reliable — flag it and let the rule skip it.

Fever or illness is the big one — an infection raises your temperature for reasons unrelated to your cycle and can paint a convincing false shift. Alcohol the night before commonly raises the next morning's reading. Short or broken sleep, a very different wake time, travel across time zones, and some medications also blur the picture. That's why this tool has flags for illness, alcohol, and poor sleep: flagged days still appear on your chart, but the rule engine excludes them, so one feverish morning can't manufacture an ovulation date. When in doubt, flag it — a slightly sparser chart beats a misleading one.

Not on its own, and we want to be direct about that. Temperature only confirms ovulation after it happened, so it can't warn you the fertile window is opening — the days you'd most need to know about. ACOG groups temperature charting with the fertility-awareness-based methods, which as typically used have some of the higher failure rates among contraceptive methods: ACOG puts typical-use fertility-awareness failure at roughly 12 to 24 pregnancies per 100 people in the first year. People who use fertility awareness to avoid pregnancy combine several signs, get proper instruction, and accept the trade-offs. If preventing pregnancy matters to you, talk with a provider about methods that fit your life — don't rely on this chart alone.

Yes, and we treat that seriously on purpose. Your entries are stored only in this browser on this device, using local storage — they are never sent to a server, tied to an account, or shared, and using the tool tells us nothing about you. The 'Delete my chart' button wipes everything instantly, and the export button just creates a plain-text file on your own device. In a post-Dobbs environment we think cycle and ovulation data deserve that care by default. This is general educational information, not medical advice or a diagnosis, and it can't replace your clinician.