BBT Calculator
Analyze basal body temperature (BBT) shift to identify the likely ovulation day in a menstrual cycle.
Formula
Shift = post-ovulation BBT − pre-ovulation avg BBT; Confirmed if shift ≥ 0.2°F
- Ovulation typically occurs the day before the temperature rise.
- Cover line = pre-ovulation average + 0.1°F.
- A confirmed shift of ≥0.2°F above cover line for 3 consecutive days confirms ovulation.
- BBT must be taken at the same time each morning before getting out of bed.
Pre-ovulation avg 97.6°F, rise to 98.2°F on day 14
Inputs
- Pre-Ovulation Avg BBT: 97.6 °F
- Post-Ovulation BBT (last 3 days): 98.2 °F
- Cycle Day of Temperature Rise: 14
Shift = 0.6°F. Cover line = 97.7°F. Rise confirmed. Ovulation likely occurred on day 13.
Frequently asked questions
How do I measure BBT?
Use a basal thermometer measuring to 2 decimal places. Take temperature first thing in the morning, same time every day.
What is a confirmed BBT shift?
Three consecutive temperatures at least 0.2°F above the pre-ovulation average cover line.
Does illness affect BBT?
Yes — fever, poor sleep, alcohol, and travel can all elevate BBT regardless of ovulation.
Can BBT tell me before ovulation?
No — BBT confirms ovulation after it happens. Combine with cervical mucus tracking for forward-looking prediction.