About the Basal Body Temperature Chart
This basal body temperature (BBT) chart turns a list of your daily waking temperatures into a cycle chart and looks for the thermal shift — the small, sustained rise of about 0.2–0.5 °C (0.4–1 °F) caused by progesterone after ovulation. When it finds the shift, it draws the coverline, tells you the cycle day ovulation most likely happened, and shows how long your temperatures have stayed high.
It is for anyone tracking fertility signs: people trying to conceive who want to confirm they are ovulating and learn their pattern, and people using fertility awareness who want a second check on their own chart. Enter one reading per day starting from cycle day 1 (the first day of your period), in °F or °C, and mark a missed day with a dash.
The shift is detected with the widely taught “3 over 6” rule: three consecutive temperatures higher than all of the previous six, with the third at least 0.2 °C (0.36 °F) above that highest low. BBT only confirms ovulation after it has happened, so it cannot predict your fertile days in the current cycle on its own; several months of charts reveal your usual ovulation day. Illness, alcohol, poor sleep and taking your temperature at a different time can all cause misleading spikes. This chart is not a contraceptive method on its own.
How to use the basal body temperature chart
- 1Take your temperature right after waking, before getting up, at the same time each day.
- 2Enter each day’s reading from cycle day 1, using - for any day you missed.
- 3Choose °F or °C and enter the first day of your period.
- 4Read the result: the coverline, shift day and estimated ovulation day.
- 5Chart for 2–3 cycles to learn your usual ovulation day.
Formula and method
The calculator scans your readings for the first day d where the next three temperatures are all higher than every one of the six readings before them (the “3 over 6” rule), and the third high is at least 0.2 °C (0.36 °F) above the highest of those six. The highest of the six is the coverline. The six-day window must have no missed readings, and the three highs must be on consecutive days.
Ovulation most often happens on the last low-temperature day, the day before the first high, so that is reported as the estimated ovulation day; in reality it can fall a day or two either side. The next period is estimated 14 days after ovulation, a typical luteal phase. The rise is the average of the high readings minus the average of the low readings.
- T[d]
- Basal temperature on cycle day d
- Coverline
- Highest of the six readings before the shift
Worked examples
Typical 28-day chart in °F
The highest of days 9–14 is 97.4 °F. Days 15, 16 and 17 (97.7, 97.8, 97.9) are all above it and the third is more than 0.36 °F higher, so the shift starts on day 15 and ovulation most likely happened on day 14 (September 14). Lows average 97.26 °F and highs 97.96 °F — a rise of about 0.69 °F.
°C chart with a clear shift
The six readings before day 13 top out at 36.5 °C. Days 13–15 read 36.6, 36.7 and 36.8, all higher, with the third 0.3 °C above the coverline, so ovulation is estimated on cycle day 12. The highs average 36.76 °C against 36.39 °C for the lows, a rise of about 0.37 °C.
A borderline rise that only confirms a day later
Days 11–13 (97.5, 97.6, 97.7) beat the previous high of 97.4, but the third is only 0.3 °F higher, short of the 0.36 °F rule. Starting a day later, the coverline becomes 97.5 and days 12–14 (97.6, 97.7, 97.9) qualify, so the shift is dated to day 12 with ovulation around day 11.
Too early in the cycle
With only 8 readings the 3-over-6 rule cannot be applied — it needs six earlier readings plus three highs — so keep charting each morning.
Frequently asked questions
What is a normal basal body temperature?+
Before ovulation BBT is typically about 36.1–36.4 °C (97.0–97.5 °F). After ovulation progesterone raises it by roughly 0.2–0.5 °C (0.4–1.0 °F), and it stays higher until your next period.
Can BBT predict ovulation?+
Not in the current cycle — the rise appears after ovulation has already happened. By charting several cycles you can see which day you usually ovulate and predict future fertile windows, ideally alongside cervical mucus or ovulation tests.
What is a coverline on a BBT chart?+
It is a horizontal line drawn at the highest of the six temperatures just before the shift. Post-ovulation temperatures should stay above it, which makes the two phases of the cycle easy to see.
How do I take my basal body temperature correctly?+
Use a basal thermometer reading to two decimals, take it right after waking after at least 3 hours of sleep, before sitting up, talking or drinking, and at about the same time every day.
Does a high BBT mean I am pregnant?+
Temperatures that stay above the coverline for 18 days or more can be an early sign of pregnancy, because progesterone stays high. A home pregnancy test after your missed period will confirm it.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.