Basal Body Temperature & Ovulation Detection
Track your daily basal body temperature to detect the biphasic pattern that indicates ovulation. Natural fertility awareness tracking method.
See step-by-step calculation
When to use this calculator
- You want to confirm whether you ovulated this cycle.
- You use the symptothermal method to plan or prevent pregnancy.
- You want to confirm ovulation after a positive LH test.
- You track fertility and want to analyze your temperature pattern.
- You suspect you're not ovulating and want data to discuss with your doctor.
BBT Patterns and Their Meaning
| BBT Phase | Typical Range (°C) | What It Indicates |
|---|---|---|
| Pre-ovulatory (follicular) | ~36.1–36.4°C | Low phase; ovulation has not yet occurred |
| Post-ovulatory shift | +0.3–0.5°C above baseline | Confirms ovulation already occurred (progesterone rise) |
| Post-ovulatory (luteal) | ~36.4–36.8°C | Sustained high phase; normal luteal phase |
| Elevated > 16 days | Sustained high | Possible pregnancy |
| Biphasic pattern | Clear low → high shift | Ovulatory cycle confirmed |
| Monophasic pattern | No clear shift | Possible anovulatory cycle |
| Sharp drop after high phase | Falls toward pre-ovulatory level | Menstruation likely imminent |
Fuente: ACOG – Fertility Awareness-Based Methods of Family Planning (2019); valores de referencia tomados del contenido de la calculadora.
How it works
How Basal Body Temperature Works
Before ovulation, estrogen dominance keeps your basal body temperature (BBT) lower (~36.1–36.4°C / 97.0–97.5°F). After ovulation, the corpus luteum releases progesterone, which acts on the hypothalamus to raise your resting temperature by ~0.2–0.5°C (~36.4–36.8°C / 97.6–98.2°F). This creates a biphasic pattern — a visible two-phase graph split by the thermal shift.
The luteal phase (post-ovulation) typically lasts 12–16 days in most people with ovulatory cycles. This window is relatively stable for a given individual, which is what makes BBT charting useful over time.
How This Calculator Works
The calculator identifies your thermal shift by comparing your post-ovulation readings against a coverline — a reference line drawn 0.05°C above your highest temperature in the previous 6 low-phase days. Ovulation is confirmed when 3 consecutive temperatures stay above the coverline. This is the standard rule used in the Fertility Awareness Method (FAM) and validated in clinical literature on natural family planning.
From that confirmed ovulation day, the calculator estimates:
How to Measure Correctly
1. Measure immediately upon waking, before getting out of bed, talking, or using the bathroom.
2. Use a BBT-specific thermometer with at least 0.01°C (two decimal) precision — standard fever thermometers are not sensitive enough.
3. Measure at the same anatomical site every day. Rectal and vaginal temps run ~0.3–0.5°C higher than oral and are considered more consistent.
4. Measure at the same time daily (within ±30 minutes). Each hour of deviation can shift readings by ~0.1°C.
5. You need at least 3–4 hours of uninterrupted sleep before measuring for the result to be valid.
6. Record readings immediately — memory drift introduces charting errors.
Interpreting Your Chart
| Pattern | Likely Meaning |
|---|---|
| Biphasic (clear low → high shift) | Ovulation occurred |
| Monophasic (flat, no shift) | Possible anovulatory cycle |
| Elevated > 18 days (no period) | Early pregnancy sign — consider a test |
| Mid-luteal drop then rise ("implantation dip") | Anecdotal; not clinically validated |
| Sharp drop after sustained high temps | Menstruation likely within 12–24 hours |
A luteal phase under 10 days (short high phase) may indicate luteal phase deficiency, which can affect implantation — worth discussing with a gynecologist if consistently observed.
What BBT Confirms vs. What It Cannot Do
BBT is a retrospective indicator. The thermal shift happens after the egg has already been released. This means:
Common Errors That Distort Readings
Best Used as Part of a Combined Approach
BBT alone identifies the post-ovulation phase. To also identify your pre-ovulatory fertile window, combine it with:
This combination is the basis of the Sympto-Thermal Method, which has been studied more rigorously than BBT alone.
Related Calculators
Example: 36.7°C on cycle day 16
Frequently asked questions
What's a normal basal body temperature?
How much does BBT rise after ovulation?
What if my temperature doesn't rise?
Can elevated basal body temperature indicate pregnancy?
What thermometer should I use?
What can affect my basal body temperature reading?
Is BBT alone reliable for contraception?
How long does BBT stay elevated after ovulation?
Why measure BBT immediately upon waking?
Sources & references
Methodology & trust
Calculadora de salud revisada por el equipo editorial de Hacé Cuentas, contrastada con ACOG — Fertility Awareness-Based Methods, según nuestra política editorial y metodología.
Última revisión: June 22, 2026. Los parámetros se verifican periódicamente con las fuentes citadas.
Calculations run 100% in your browser. We do not store or transmit your data.
Indicative results. For critical decisions, consult a professional.
Rodríguez, M. (2026). Basal Body Temperature & Ovulation Detection. Hacé Cuentas. https://hacecuentas.com/basal-body-temperature-ovulation
Contenido bajo licencia CC-BY 4.0 — reutilizable citando la fuente con enlace a Hacé Cuentas.