Basal Body Temperature Shift Calculator
After ovulation, basal body temperature rises slightly and stays elevated through the luteal phase. This calculator measures that thermal shift by subtracting your average pre-ovulation temperature from your average post-ovulation temperature. Enter the two averages in Fahrenheit or Celsius and it reports the shift, including a rough guide to whether the rise looks clear. This is an educational cycle-tracking tool. A full multi-day chart and professional advice give a more reliable picture than any single number.
Temperature shift formula
Shift = average post-ovulation temperature - average pre-ovulation temperature
The shift is simply the difference of the two averages in your chosen unit. The assessment label uses a clear-shift threshold of about 0.4 degrees Fahrenheit, or about 0.2 degrees Celsius, as a rough descriptive guide only.
Worked example
Pre-ovulation average 97.4 degrees F, post-ovulation average 98.0 degrees F:
- Shift = 98.0 - 97.4 = 0.60 degrees F.
- Assessment: clear shift (at or above 0.40 degrees F).
BBT shift: frequently asked questions
What is the basal body temperature shift?
After ovulation, rising progesterone causes a small sustained rise in resting (basal) body temperature, typically on the order of about 0.5 to 1.0 degrees Fahrenheit (roughly 0.3 to 0.5 degrees Celsius). Comparing your average temperature before ovulation with your average after it reveals this shift, which is a retrospective sign that ovulation has occurred.
How do I measure basal body temperature?
Take your temperature with a sensitive thermometer immediately on waking, before getting up, at the same time each morning after at least a few hours of sleep. Record the daily values, then average the readings from the days before your suspected ovulation and the days after. This calculator subtracts the two averages.
Does a temperature shift predict ovulation in advance?
No. The shift confirms ovulation after it has happened, because the temperature rises in response to ovulation rather than before it. To anticipate ovulation ahead of time, ovulation predictor kits or cervical mucus tracking are more useful. Charting temperature over several cycles still helps confirm a consistent ovulation pattern.
What counts as a clear shift?
Many charting methods look for a sustained rise of at least about 0.4 degrees Fahrenheit (around 0.2 degrees Celsius) above the previous baseline, held for three or more days. This calculator simply reports your measured difference; interpreting whether it is a clear, sustained shift is best done with a full chart and, if needed, professional guidance.
Official sources
- Office on Women's Health, U.S. Department of Health and Human Services: Your menstrual cycle.
- U.S. National Library of Medicine, MedlinePlus: Ovulation.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.