Basal Body Temperature: Charting, Ovulation Shift & Accuracy
What is basal body temperature?
Basal body temperature (BBT) is your body's lowest resting temperature, measured immediately after waking and before any activity. It typically runs 97.0–97.7°F (36.1–36.5°C) before ovulation. After ovulation, the hormone progesterone nudges it up by 0.5–1.0°F (0.3–0.6°C), where it stays until the next period. That small, sustained shift is what BBT charting detects.
Important framing: the shift appears after ovulation has already happened, so BBT confirms the pattern of a cycle rather than predicting the fertile window in advance. Clinicians consider it a helpful tracking tool, not a contraceptive method on its own.
How to measure BBT correctly
The rules that make the chart readable
- Measure at the same time every morning, ideally within 30 minutes, after at least 3 hours of sleep.
- Before anything else - before sitting up, talking, or checking your phone. Movement raises the reading.
- Use a basal thermometer (two decimal places). Regular thermometers round away the shift you are looking for.
- Same method every day - oral readings and readings from wearables are not interchangeable mid-cycle.
- Mark the noisy days: alcohol, illness, poor sleep and travel all distort a single morning's number.
Reading the chart
| Cycle phase | Typical BBT | What it means |
|---|---|---|
| Menstruation to ovulation | 97.0–97.7°F (36.1–36.5°C) | Low, fairly flat baseline |
| Ovulation day (approx.) | Often a small dip, then rise | The shift begins |
| After ovulation | 97.6–98.6°F (36.4–37.0°C) | Sustained 0.5–1°F above baseline |
| 3+ high days in a row | Above the previous 6-day maximum | Ovulation confirmed (the 3-over-6 rule) |
| Just before the period | Falls back toward baseline | Progesterone declining |
| 18+ high days, no period | Stays elevated | Worth taking a pregnancy test |
BBT vs. everyday body temperature
BBT is deliberately measured at its daily minimum, so it runs about a degree below the familiar daytime readings covered in our normal body temperature guide. A BBT chart cannot diagnose anything by itself - it is one signal, best combined with other fertility signs or apps, and worth discussing with a clinician if cycles look irregular. This page is general information, not medical advice.
Wearables vs. the morning thermometer
Rings, bracelets and patches now track temperature continuously through the night, which removes the biggest source of BBT noise: inconsistent wake-up measurement. Studies show nighttime wearable readings detect the post-ovulation shift at least as reliably as disciplined oral charting - and far better than undisciplined charting.
Two caveats: wearables measure skin temperature, which runs cooler and swings more with the bedroom environment than core temperature (keep the room steady - see the ideal sleeping temperature), and their absolute numbers are not comparable with an oral thermometer’s. Pick one system and judge only its own trend.
Reading the tricky charts
- Flat chart, no shift: an occasional anovulatory cycle (no egg released) is normal; several in a row are worth discussing with a clinician.
- Short high phase: if temperatures fall back within 10 days of the rise, the luteal phase may be short - another conversation-starter, not a diagnosis.
- Jagged readings: alcohol, illness, travel and broken sleep each distort single mornings. Chart around them rather than reading meaning into one spike.
- Slow-rise patterns: not everyone gets a crisp step; a staircase over 2-3 days is common and still counts once the 3-over-6 rule is met.