Basal Body Temperature Shift Calculator

How it is calculated

Enter two averages: the mean of your waking temperatures before the suspected rise (the low phase) and the mean after it (the high phase). The tool computes shift = post average − pre average, in °C, and compares it with a fixed threshold of 0.5 °F, which is 0.28 °C: ACOG describes the post-ovulation rise of basal temperature as 0.5 to 1 °F, so the tool takes the lower end. A shift of 0.28 °C (0.5 °F) or more is labelled "Likely ovulated"; anything smaller, including a negative shift, is "No clear shift". Values are read as Celsius (30–45). If both are clearly Fahrenheit (86–113), they are accepted and the difference is converted with °C = °F × 5/9, so the shift is always shown in °C. Anything else, or one Celsius and one Fahrenheit value, shows no result. The tool does not find the day of ovulation or check how many days you averaged.

Common mistakes

• Mixing units: one average in °C and the other in °F gives no result. Enter both in the same scale.
• Averaging the wrong days. The low average should cover days before the rise and the high average days after it; including transition days or the period itself blurs the difference.
• Using temperatures taken at different times or after getting up. Basal temperature has to be taken at the same time, straight after waking; illness, alcohol or a poor night can distort single readings.
• Treating the result as proof of fertility, or as contraception. It only suggests that ovulation probably happened already.
• Entering a daytime or fever reading. Values outside 30–45 °C (86–113 °F) are not accepted as body temperatures.