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Basal Body Temperature Calculator (BBT Coverline & 3-over-6 Rule)

Enter 6 low basal temperatures and the next 3 readings to draw your BBT coverline and check the 3-over-6 rule that confirms ovulation has happened, in °C or °F.

Thermometer unit
Low day 1
Low day 2
Low day 3
Low day 4
Low day 5
Low day 6
First higher reading
Second higher reading
Third higher reading
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How it works

Coverline = highest of the 6 low readings · Shift confirmed when 3 consecutive readings are all above the coverline AND the third is ≥ 0.2 °C (0.4 °F) above it

After ovulation, progesterone raises the waking (basal) body temperature by about 0.2–0.5 °C (0.4–1.0 °F) until the next period. The sympto-thermal “3-over-6” rule, described in the WHO family planning handbook, looks at the 6 readings before the rise: when 3 consecutive readings are higher than all 6, and the third is at least 0.2 °C above the highest of them, ovulation has passed. BBT only confirms ovulation after the fact, so it cannot predict the fertile days on its own; take it at the same time each morning before getting up, with a 2-decimal thermometer.

Worked example

Six low readings of 36.30, 36.40, 36.20, 36.40, 36.30 and 36.35 °C give a coverline of 36.40 °C. The next three are 36.55, 36.60 and 36.70 °C — all above 36.40, and the third is 0.30 °C above it (≥ 0.2), so the thermal shift is Confirmed. The 3 high days average 36.62 °C against 36.32 °C for the low days, a rise of about 0.29 °C, and ovulation was probably the day before the 36.55 reading.

Sources: WHO & Johns Hopkins CCP (2018) Family Planning: A Global Handbook for Providers — Fertility awareness methods (basal body temperature) · NHS — Natural family planning (fertility awareness)

Frequently asked questions

What is a coverline on a BBT chart?

A horizontal line drawn just at or above the highest of the 6 temperatures before the rise. Temperatures after ovulation should stay above it until your next period.

How much does temperature rise after ovulation?

Usually 0.2–0.5 °C (0.4–1.0 °F). The rise can be sudden or creep up over a few days; what matters is that the higher level is sustained.

Can basal body temperature show pregnancy?

If the higher temperatures continue for 18 or more days after ovulation, pregnancy is likely — take a pregnancy test. A dip in the middle of the high phase does not reliably signal implantation.

What can throw off my BBT?

Fewer than 3–4 hours of sleep, measuring at a different time, alcohol, illness or fever, travel across time zones and some medications. Mark those days and leave them out of the 6 low readings.

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This calculator is for education only and is not a diagnosis or a method of contraception you can rely on by itself. Talk to your doctor, nurse or midwife about your cycle, bleeding or family planning.