BBT (Basal Body Temperature) Calculator
Track and chart your basal body temperature to detect ovulation patterns, identify fertile windows, and analyze menstrual cycle phases with precision.
| Day | Date | Temp | Time | Menses | Mucus | Intercourse | Notes | Actions |
|---|
About
Basal body temperature (BBT) is the lowest resting core temperature, measured immediately upon waking before any physical activity. After ovulation, progesterone elevates BBT by approximately 0.2 - 0.5 °C (0.4 - 0.9 °F), creating a biphasic pattern visible only through consistent daily measurement. This thermal shift confirms ovulation retrospectively. Misidentifying the shift or using inconsistent timing introduces error that can invalidate weeks of data. This tool applies the standard Fertility Awareness Method "3 over 6" rule: ovulation is confirmed when 3 consecutive temperatures exceed the highest of the previous 6 readings by at least 0.1 °C.
The calculator generates a coverline, identifies cycle phases, and renders a clinical-grade chart. It does not replace medical consultation. Temperature accuracy depends on measurement consistency: same time (±30 min), same method (oral/vaginal), after minimum 3 hours uninterrupted sleep. Illness, alcohol, disrupted sleep, and travel across time zones can produce aberrant readings that this tool cannot automatically correct.
Formulas
The coverline establishes a baseline temperature threshold. It is computed from the 6 pre-ovulatory temperatures immediately before the suspected thermal shift.
Ovulation is confirmed when 3 consecutive readings satisfy:
This is the standard "3 over 6" rule from the Fertility Awareness Method (FAM). The thermal shift day is designated as the first day Ti exceeds the coverline.
Temperature unit conversion between Celsius and Fahrenheit uses:
Luteal phase length is calculated as:
Where T = temperature reading, i = day index in cycle, D = calendar day, L = phase length in days. A luteal phase shorter than 10 days may indicate luteal phase deficiency and warrants clinical evaluation.
Reference Data
| Cycle Phase | Typical Duration | BBT Range (°C) | BBT Range (°F) | Dominant Hormone | Key Indicator |
|---|---|---|---|---|---|
| Menstruation | 3 - 7 days | 36.1 - 36.4 | 97.0 - 97.5 | Low estrogen & progesterone | Bleeding |
| Follicular (pre-ovulation) | 7 - 21 days | 36.1 - 36.4 | 97.0 - 97.5 | Rising estrogen | Lower, stable temps |
| Ovulation (thermal nadir) | 1 day | 35.9 - 36.2 | 96.6 - 97.2 | LH surge | Possible temp dip |
| Luteal (post-ovulation) | 10 - 16 days | 36.4 - 36.8 | 97.5 - 98.2 | Rising progesterone | Sustained elevated temps |
| Late Luteal / Pre-menstrual | 1 - 3 days | 36.3 - 36.6 | 97.3 - 97.9 | Falling progesterone | Temp drop before period |
| Implantation Dip (if pregnant) | 1 day (DPO 7 - 10) | −0.1 to −0.3 | −0.2 to −0.5 | Estrogen secondary surge | Single-day dip then rise |
| Triphasic Pattern (possible pregnancy) | DPO 7 - 12+ | 36.7 - 37.0+ | 98.1 - 98.6+ | Sustained progesterone | Second temp rise in luteal |
| Anovulatory Cycle | Variable | 36.1 - 36.5 | 97.0 - 97.7 | No progesterone rise | No biphasic shift |
| Short Luteal Phase (<10 days) | <10 days | Normal luteal range | Normal luteal range | Insufficient progesterone | Early temp drop |
| Normal Full Cycle | 21 - 35 days | Biphasic | Biphasic | Estrogen → Progesterone | Clear thermal shift |
| Fertile Window | 5 - 6 days | Pre-shift temps | Pre-shift temps | Peak estrogen + LH | Egg white cervical mucus |
| Cervical Mucus: Dry | Post-menstrual | N/A | Low estrogen | Infertile | |
| Cervical Mucus: Sticky | Early follicular | N/A | Rising estrogen | Possibly fertile | |
| Cervical Mucus: Creamy | Mid follicular | N/A | Moderate estrogen | Fertile | |
| Cervical Mucus: Egg White (EWCM) | Peak fertility | N/A | Peak estrogen | Most fertile | |