BBT Calculator: Basal Body Temperature Charting Tool
Log your daily basal body temperature readings and let this tool plot your chart, find your coverline, and estimate the day your temperature shift points to ovulation.
Medically reviewed by Charlotte Rose, RN · Reviewed for accuracy: July 2026 · Based on the three-over-six charting rule used in fertility awareness methods
Log your daily readings
Take your temperature at the same time each morning, before you get out of bed, talk, or drink anything. If a reading was disturbed (alcohol, illness, a late night, travel), flag it in the note column — flagged days are still charted but are skipped when the coverline is calculated.
| Day | Temp | Note |
|---|
Your chart
Add at least seven daily readings, then tap “Chart my cycle” to see your coverline, estimated ovulation day, and cycle phases.
How the three-over-six rule works
A single high or low temperature does not mean much on its own — basal body temperature drifts by small amounts for all sorts of reasons. What matters is the pattern. This calculator uses the three-over-six rule, the same convention used across most fertility awareness charting systems: it looks at the six valid temperatures immediately before a rise, finds the highest one, and checks whether the next three readings all land above it, with at least one of those three at least 0.2°F higher. When that condition is met, the day the rise began is marked as your thermal shift, and the highest of the six prior temperatures becomes your coverline — the reference line separating your lower, pre-ovulation temperatures from your higher, post-ovulation ones.
Quick answer
To read your own chart by hand: find the highest temperature among the six days before a rise. If the next three days in a row sit above that number, draw your coverline at that highest prior value. The first of those three elevated days is your estimated thermal shift, and ovulation is presumed to have happened one to two days before it.
Reading your BBT chart phase by phase
| Phase | What you will see | Typical length |
|---|---|---|
| Follicular phase | Lower, fairly flat temperatures before ovulation | Variable — often 10 to 16 days |
| Ovulation | Sometimes a small dip, then the rise begins | 1 to 2 days before the shift |
| Thermal shift | Three or more readings in a row above the coverline | Confirms the shift, does not predict it |
| Luteal phase | Higher, more stable temperatures after the shift | Usually 10 to 16 days |
| Next period | Temperatures fall back toward baseline | If no pregnancy occurs |
What a triphasic pattern means
Most cycles show a simple two-step, or biphasic, shape: lower before ovulation, higher after. Occasionally a chart shows a second, smaller rise partway through the luteal phase — a triphasic pattern. Some people notice this around the time implantation is thought to occur, but a triphasic shape is not a pregnancy test and plenty of triphasic charts are not pregnant, while plenty of pregnant charts never show one. This tool will flag the pattern for your own reference only, described in plain terms rather than as a result. If you are trying to confirm a pregnancy, our Pregnancy Due Date Calculator and a home test are the more reliable next steps.
Why flagged readings are excluded from the coverline
Alcohol, illness, poor sleep, a late bedtime, or a temperature taken well outside your usual time can all push a single reading up or down without reflecting a real hormonal shift. Flagging a day keeps that data point visible on your chart, styled differently, while leaving it out of the six-temperature comparison the coverline is built from — so one rough night does not throw off the whole read.
A worked example
Say your first six charted temperatures run 97.2, 97.4, 97.3, 97.2, 97.4, 97.3°F. The highest of those is 97.4°F. If your next three readings come in at 97.9, 98.0, and 97.9°F, all three sit above 97.4°F and one is more than 0.2°F higher — so the shift is confirmed. Your coverline is drawn at 97.4°F, the thermal shift lands on the first of those three elevated days, and ovulation is estimated one to two days earlier.
Related maternal health calculators
This tool is part of the Maternal Health Calculators hub. These companion tools cover cycle tracking, ultrasound dating, and maternity planning — each answers a different question, so pick whichever fits where you are.
Frequently asked questions
Basal body temperature is your body’s lowest resting temperature, taken first thing in the morning before you get up, eat, or drink. Before ovulation, estrogen keeps it relatively low. After ovulation, progesterone from the corpus luteum raises it by roughly 0.2 to 0.5°F, and it stays elevated through the luteal phase until your next period, when it typically drops again.
The three-over-six rule needs at least six pre-shift readings plus three elevated readings after them, so seven to ten consistent days is the practical minimum. A full cycle of daily readings gives a much clearer picture, and charting two or three cycles back to back shows you your personal pattern rather than a single month’s noise.
The coverline is a reference value, not a prediction. It is set at the highest of the six temperatures right before your rise, so every reading below it belongs to the lower, pre-ovulation range and readings above it belong to the higher, post-ovulation range. It only appears once three confirming days have shown up.
Not exactly. Temperature only rises after ovulation has already happened, once progesterone starts being produced, so BBT confirms that ovulation occurred rather than predicting it in advance. It typically lands one to two days behind the actual event. If you want to anticipate ovulation ahead of time, pairing BBT with ovulation predictor kits or cervical mucus tracking is more reliable.
Alcohol, a fever, poor sleep, or a temperature taken at an unusual time can all push one reading off your normal pattern without meaning anything hormonally. Flagging that day keeps it visible on the chart but leaves it out of the six-day comparison the coverline is built from, so a single rough night does not distort your read.
A triphasic pattern is a second, smaller temperature rise partway through the luteal phase, after the initial post-ovulation shift. Some charters notice it around the time implantation is thought to happen, but it is not a diagnostic sign — many triphasic charts are not pregnant and many pregnancies never show one. A home pregnancy test remains the reliable way to confirm.
Yes. Use the °F / °C toggle above the table to switch units at any time — your entered readings convert automatically and the chart, coverline, and shift detection all recalculate in the unit you have selected.
Basal body temperature alone confirms ovulation after it has already occurred, so it only marks the end of your fertile window, not the beginning. Fertility awareness methods used for contraception typically combine BBT with cervical mucus observation and cycle-length calculations, and are best learned through a certified instructor rather than a single online tool.
Not necessarily. Anovulatory cycles, illness, irregular sleep, inconsistent timing, or simply a naturally subtle biphasic pattern can all keep the three-over-six rule from triggering. A few flat cycles in a row are worth mentioning to your provider, but one unclear chart is common and not on its own a cause for concern.
Methodology
This tool applies the three-over-six rule used in fertility awareness charting: it scans each set of six consecutive, non-flagged temperature readings and finds the highest value among them. It then checks the three readings that immediately follow. If all three sit above that highest prior value, and at least one of the three is 0.2°F (about 0.1°C) or more above it, the first of the three elevated readings is marked as the thermal shift and the highest of the six prior readings becomes the coverline. Estimated ovulation is shown as one day before the confirmed shift. Flagged readings (illness, alcohol, disrupted sleep, off-schedule timing) are still plotted on the chart but are excluded from the six-day comparison window. A secondary check looks for a further sustained rise of 0.2°F or more within the luteal phase to flag a possible triphasic pattern for the user’s own reference. All results are educational estimates based on self-reported data, not a clinical or diagnostic test, and are not a substitute for guidance from a qualified provider or a certified fertility awareness instructor.
Charlotte Rose, RN
A professional nurse turned SEO strategist and web designer with 10+ years of experience in the online health field. Charlotte reviewed this calculator’s charting logic and educational content for clinical accuracy and clarity.
References & trusted sources
- NIH / PMC — Modern fertility awareness methods and the BBT three-over-six rule
- NIH / PMC — Identifying ovulation via the three-over-six criterion
- ACOG — Fertility Awareness-Based Methods of Family Planning
- NICHD — The Menstrual Cycle
- MedlinePlus — Basal body temperature
This calculator provides educational estimates based on the temperatures you enter and is not a medical test, diagnosis, or method of contraception. Individual cycles vary, and this tool cannot account for every factor that affects basal body temperature. If you are trying to conceive, avoid pregnancy, or have concerns about your cycle, please speak with your healthcare provider or a certified fertility awareness instructor. Last reviewed & updated: July 2026.