BBT Calculator

Basal body temperature (BBT) is your body temperature at rest, taken as soon as you wake up. After ovulation, the hormone progesterone raises it slightly and it stays up until your next period. This calculator looks for that rise, called the thermal shift, in the readings you enter.

What a BBT Chart Measures

A BBT chart shows whether your temperature has moved from a lower level to a higher one. It does not measure hormones and it does not predict ovulation. It can only show that ovulation has already happened¹.

The Formula

This calculator uses the "three over six" rule, where the shift is found when "three consecutive daily readings are higher than the six preceding daily temperatures"¹:

Coverline = the highest of the 6 readings before the first high reading

Shift confirmed = 3 readings in a row above the coverline, with the third at least 0.2 °C (0.36 °F) above it

Estimated ovulation day = the last low day before the first high reading

Luteal phase so far = last day entered minus estimated ovulation day

The 0.2 °C check matches the low end of the usual 0.2 to 0.5 °C rise². Placing ovulation on the last low day follows a study of more than 600,000 cycles³.

How This Calculator Computes Your Result

Choose °F or °C and enter one waking temperature per cycle day, starting on day 1 (the first day of full period flow). Adding your cycle start date shows calendar dates. Leave missed days blank, and tick "unusual" for readings after illness, alcohol, short or broken sleep, or a very different waking time. Blank and ticked days are skipped, as are fever readings of 100.4 °F (38.0 °C) or higher.

You need at least 9 usable readings (6 low, 3 high). If the third high reading is under 0.2 °C above the coverline, one more high reading confirms a smaller rise. You see the coverline, shift day, estimated ovulation day, days since then, and average low and high temperatures (to 2 decimals), plus a chart.

What Your Result Means

The size of your rise is also shown. A rise of 0.2 to 0.5 °C (about 0.4 to 1.0 °F) is the usual range². A much larger rise can be a sign that an unusual reading slipped in.

Your estimated ovulation day is a best guess. The rise can start from 1 day before to 3 days after ovulation⁴, so the calculator also shows a possible range. In research cited by one study, the chart's lowest point matched the ovulation day in only 43% of cycles¹.

Days since ovulation is your luteal phase so far. Most luteal phases last 7 to 17 days, with an average of 12.4³. If your temperature stays high for 18 days or more, the calculator suggests a pregnancy test. When your period starts, the Luteal Phase Calculator measures the full luteal phase.

How to Measure Correctly

• Take your temperature right after you wake, before you get out of bed or eat anything².

• Measure at about the same time each morning, after at least 6 hours of unbroken sleep⁴.

• Use the same place every day (mouth, vagina or rectum) and a sensitive thermometer⁴.

• Enter the number exactly as shown. Do not round it.

Thermometer resolution matters because the rise is small¹. A thermometer that reads in 0.1 °C steps shows the smallest normal rise as just two steps. A basal thermometer that reads to two decimals gives a clearer chart.

What Can Throw Off a Reading

Alcohol, late nights or sleeping in, broken sleep, travel and time zone changes, shift work, stress, illness and some medicines can all change a reading⁴. A fever makes BBT hard to use for that stretch². Like a large app study³, this calculator leaves out flagged readings. One unusual high reading among your six lows can raise the coverline and delay the shift by a day or more.

Some tools adjust a reading for the time you took it, often by 0.1 °C per hour. We do not, because we found no study behind that correction. At that rate a 2 hour difference equals 0.2 °C, the whole minimum rise, so a wrong correction could create or hide a shift. Mark a reading taken at a very different time as unusual instead.

Can BBT Be Used as Birth Control?

Not on its own. A World Health Organization family planning handbook states that reliable effectiveness rates are not available for the BBT method alone². Across fertility awareness methods as commonly used, about 15 in 100 women get pregnant in the first year². The NHS puts fertility awareness methods at 91 to 99% effective with perfect use, but 76% with typical use⁶.

The symptothermal method, which adds cervical mucus signs to BBT, does better: under 1 pregnancy per 100 women in a year with correct use and 2 as commonly used². In a large study of one such method, 1.8 in 100 women had an unintended pregnancy over 13 cycles, and 0.6 in 100 when couples did not have unprotected sex in the fertile time⁵. This calculator is not that method and does not mark any day as safe.