Estimate your ovulation day and fertile window from a short BBT log. You’ll also see confidence, DPO, next period, earliest reliable test day, and a temperature chart.
Methodology & Sources
What this estimates
Based on your basal temperatures, optional OPK positives, and a chosen luteal length, the tool estimates an ovulation day with a ± window, a fertile window, DPO today, the next period estimate, and the earliest reliable pregnancy test day. A small chart shows your temps, the auto coverline, and OPK markers.
How BBT detection works here
- Clean data: Disturbed readings (poor sleep, illness, alcohol, time shifts) are ignored for detection.
- Coverline: For each candidate day, the tool looks back at the previous six clean temps and sets a coverline at their maximum + 0.05 °C (≈ 0.1 °F).
- 3-day rise rule: Ovulation is placed on the day before the first of three consecutive temps above the coverline by at least 0.05 °C.
- Confidence score: Starts at medium, increases to high when there are ≥8 clean lows leading into the rise or when an OPK positive falls within ±2 days of the detected ovulation day. With short logs or many disturbances it is set to low.
- Ovulation window: ±1 day when confidence is high, ±2 days when low. The result shows “most likely between …”.
Other outputs
- Fertile window: Five-day window that ends on the ovulation day (starts 5 days prior). If you select “short” cycles the start is 4 days prior; “long” cycles use 6 days prior.
- Best two next days: The next two dates inside the fertile window that are on or after today. If the window has passed, we say so.
- Next period: Ovulation + your luteal length (default 14 days, validated 10–17).
- Earliest reliable test: About 10 DPO from the detected ovulation day.
Chart specifics
- Temps are plotted as a polyline with dots; disturbed readings are greyed.
- The coverline is a dashed pink line; OPK positives appear as thin green vertical markers.
When we cannot call ovulation
If there are fewer than ~8 clean readings before a potential rise, or if there is no 3-day run above the coverline, the tool shows an “insufficient data” message with tips to keep logging and consider adding OPKs.
Limits
BBT shifts after ovulation and cannot pinpoint the moment of ovulation. Illness, alcohol, travel, and shift work can blur patterns. Educational use only. Discuss personal circumstances with a clinician.
Sources
- ACOG — Fertility Awareness-Based Methods (BBT concepts and patient guidance).
- Frank-Herrmann P, et al. Sensiplan guidelines and studies on the sympto-thermal method (coverline and rise rules).
- Ecochard R, et al. Research on timing of ovulation relative to LH surge and biomarkers.
- Stanford JB, White GL. Reviews on fertility biomarkers and timing in relation to ovulation.
What each input means
- Last period start date (optional): Day 1 is the first full-flow day. Used for context and DPO/cycle-day labels.
- BBT log (10–20 days): Enter dates and temperatures (°C/°F). Try to measure on waking, before getting up. Add disturbance flags for poor sleep, illness, alcohol, time shift.
- OPK results by date (optional): Mark negatives and any positives. Positives usually cluster 12–36 hours before ovulation.
- Luteal-phase length (optional): Default 14 days; allowed 10–17. This shifts the ovulation estimate and next-period timing.
- Typical cycle length band (optional): Used to sanity-check windows if your BBT log is short.
How to read the results
- Estimated ovulation day: A best-estimate date with a ±2–3 day window and a confidence score (low/med/high) based on how clear the rise is and any OPK support.
- Fertile window + best 2 days: The 5 days ending on the estimate. We also highlight the next two best days to try based on your inputs.
- DPO today: Days past ovulation relative to the estimate (shows “TBC” if confidence is low).
- Next period estimate: Predicted start from ovulation + your luteal length (default 14). Shown as a 3-day window.
- Earliest reliable pregnancy test day: The day after the first window end or at a chosen DPO (e.g., 12–14 DPO), depending on settings.
- Chart: Your temps plotted with an auto coverline and the detected 3-day sustained rise. OPK positives are marked.
- Insufficient data message: Shows if no sustained rise is detected. You’ll get simple tips on collecting a few more mornings of BBT and adding OPKs.
Fever, illness, alcohol, travel or broken sleep can lift temps. Flag those days so the coverline and rise detection are less affected.
Frequently asked questions
How many BBT readings do I need?
About 10–20 consecutive mornings usually gives a clear picture. Fewer is fine if a sustained rise is obvious.
What counts as a sustained rise?
At least three higher temps above the coverline after a lower run. Disturbance-flagged days may be down-weighted.
Do I have to temp at the same time?
Consistency helps. Try to take your temperature on waking, before getting up. Flag off-schedule mornings.
Should I use OPKs as well?
Yes if you can. A clear positive OPK plus a later BBT rise increases confidence in the ovulation estimate.
What if I had a cold, drank alcohol, or travelled?
Those can lift temps. Mark disturbance flags on those days so the coverline and rise detection are less affected.