Free cycle tool

BBT Coverline Calculator

The coverline is drawn through the highest of the six readings before a rise. Under the "3 over 6" rule, a shift counts once three readings in a row sit above it and the third is at least 0.2 °C (0.36 °F) higher. This calculator applies that rule to your readings and shows its working. A shift confirms ovulation after the fact; it cannot predict it, show a pregnancy or work as contraception.

Enter one cycle of waking temperatures, one per day, in °C or °F. The calculator plots them, finds the six lower readings and the higher ones the rule needs, and draws the coverline where the rule puts it. Tap any point to leave a disturbed reading out. Four example cycles show a clear rise, a slow rise, a dip and a chart with no shift.

Free, and no account.

One cycle, one chart
Clear rise Slow rise One dip No shift
Basal body temperature chart, cycle days 1 to 28No shift by this rule35.836.036.236.436.636.837.037.237.412345678910111213141516171819202122232425262728
Six lower readings Coverline Margin the third must clear Higher readings, numbered Dropped back Left out

Tap a day on the chart to leave that reading out, or to count it again. Swipe sideways on a small screen.

What the rule says

No shift by this rule

These readings never rose above a coverline and held there the way the rule needs. That is not the same as no ovulation: a temperature shift misses many ovulations.

This confirms in hindsight. A rise like this follows ovulation rather than announcing it, so it cannot say on which day ovulation happened, cannot show a pregnancy and is not contraception.

Coverline
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First higher reading
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Rule used
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One waking temperature a day, separated by commas, spaces or new lines. Type a dash for a morning you missed, so the days stay in step. Up to 45 days.

The working

Every step the rule took, with the numbers it used.

  1. All 28 counted readings were checked against the six before each one. No run of readings rose above a coverline and held there the way the rule needs.
Print the filled chart

The readings ride in the link, so it opens this same chart on another device.

This looks backwards. ACOG puts it plainly: basal body temperature "shows only when ovulation has already occurred, not when it is going to occur". A confirmed shift is a receipt, not a forecast, and the rule cannot say on which day ovulation happened.

It cannot see a pregnancy. Not a dip below the line, not a long run of high readings. A pregnancy test can: the NHS says most tests can be carried out from the first day of a missed period, and the pregnancy test calculator works out when that is.

No shift found is not "no ovulation". Measured against hormone-confirmed ovulation across 193 cycles, a temperature shift was picked up in under a quarter of them. Read several cycles together, not one flat month.

This is not contraception. Fertility awareness has to be taught properly and combines several signs; followed imperfectly it is about 76% effective, meaning 24 women in 100 get pregnant in a year, and the NHS does not officially recommend any app or device for it. Nothing on this page is medical advice.

How this is worked out

  1. A morning with no reading and any reading you tap to leave out are set aside before the rule runs, so "six in a row" means six readings that count. The study this rule comes from names fever and gaps in measurement among the things that skew an evaluation; a reading you have reason to doubt is better left out than explained away.
  2. Each counted reading is checked against the six counted readings before it. The first one higher than all six starts a possible shift, and the highest of those six is the coverline. Two more readings above the line, the last of them at least 0.2 °C (0.36 °F) above it, confirm the shift on that third day.
  3. The rule comes with two exceptions, and the page says when it used one. Slow rise: if the third higher reading is above the line but short of the 0.2 °C margin, a fourth higher reading completes the shift. Fall-back: if the second or third drops onto or below the line, it is set aside and a later reading at least 0.2 °C above the line replaces it. The two are never combined.
  4. The first run of readings that meets the rule is the answer, and every step is printed under the chart with the numbers it used. If nothing meets it, the page says so and does not guess. The rule is the one set out in full in a 2020 study of natural family planning temperature rules; other teaching traditions use variants, so this is one named rule rather than the only one.
  5. The chart is drawn to a fixed scale — 35.8 to 37.4 °C, or 96.4 to 99.4 °F — never stretched to fit your numbers, because a stretched scale makes every wobble look like a rise.
Questions

The short answers

A horizontal line drawn through the highest of the six readings just before a rise. Once it is there the shift is easy to see: the readings before it sit on or under the line, the ones after sit above it. It is worked out afterwards from readings already taken, never drawn in advance, and it is a way of reading a chart rather than a measurement of anything in the body.
Under the "3 over 6" rule, as a 2020 study of natural family planning temperature rules sets it out: find the first reading that is higher than each of the six counted readings before it. The highest of those six is the coverline. The shift is confirmed when the next two readings are also above the line and the third is at least 0.2 °C (0.36 °F) above it. This calculator does exactly that and prints each step.
Three higher readings over six lower ones. Six counted readings in a row set the level, the coverline runs through the highest of them, and three readings in a row above it — the third at least 0.2 °C higher — confirm that the temperature has shifted. There are two exceptions: a fourth higher reading may complete a slow rise, and one reading that drops back onto the line may be replaced by a later one 0.2 °C above it. They are never combined.
Yes. It is worked out fresh each cycle from that cycle's own six lower readings, so it moves when your readings move — a new thermometer, a different time of day or an ordinary difference from one month to the next all shift it. Carrying last month's line over would measure this cycle against a different one.
Any you have reason to doubt: a fever, a reading taken much later than usual, a broken night, a late drink. Tap the point on the chart and the rule runs again without it. The study this rule comes from names fever and gaps in measurement among the things that skew an evaluation. Leave a reading out because something happened that morning, never because it spoils the shape you hoped to see.
On its own, very little. One reading on or below the line straight after a rise is allowed for by the rule: it is set aside if a later reading lands at least 0.2 °C above the line. What a drop cannot tell you is whether you are pregnant, and neither can a reading that stays high. A pregnancy test can: the NHS says most tests can be used from the first day of a missed period.
No. A chart shows that a temperature rise happened, and that rise follows ovulation; it cannot see a pregnancy, and no count of high days on this page means one. The NHS says most pregnancy tests can be carried out from the first day of a missed period, and the pregnancy test calculator on this site works out that date from your cycle.
No. It means these readings did not meet this rule. Measured against hormone-confirmed ovulation across 193 cycles, a basal body temperature shift detected ovulation with a sensitivity of 0.23 — of the ovulations a hormone test confirmed, temperature found about 23 in every 100. A flat chart is weak evidence about anything. If your cycles are irregular or your pattern changes, the NHS suggests speaking to a GP.
No. This calculator is not contraception. A temperature shift shows only that ovulation has already happened, and ACOG says basal body temperature by itself is not a good way to prevent or promote pregnancy. Followed imperfectly, fertility awareness is about 76% effective — 24 women in 100 get pregnant in a year — and the NHS does not officially recommend any app or device for it. If that is your aim, talk to a doctor, nurse or pharmacist.

Where this comes from

  1. PubMed Central (2020). Continuous Body Temperature Monitoring to Improve the Diagnosis of Female Infertility. https://pmc.ncbi.nlm.nih.gov/articles/PMC7360395/
  2. American College of Obstetricians and Gynecologists (2023). Fertility Awareness-Based Methods of Family Planning. https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning
  3. PubMed Central (2022). Basal body temperature measurement and the ovulatory temperature shift. https://pmc.ncbi.nlm.nih.gov/articles/PMC9634753/
  4. PubMed Central (2018). Continuous body temperature monitoring and the fertile window. https://pmc.ncbi.nlm.nih.gov/articles/PMC6265623/
  5. PubMed Central (2021). Wrist skin temperature and basal body temperature compared against LH-confirmed ovulation. https://pmc.ncbi.nlm.nih.gov/articles/PMC8238491/
  6. NHS (2022). Doing a pregnancy test. https://www.nhs.uk/pregnancy/trying-for-a-baby/doing-a-pregnancy-test/
  7. NHS (2025). Irregular periods. https://www.nhs.uk/symptoms/irregular-periods/
  8. NHS (2024). Natural family planning (fertility awareness). https://www.nhs.uk/contraception/methods-of-contraception/natural-family-planning/

Every link above was checked when this page was last updated. Athena is not affiliated with any of these organisations, and none of them has reviewed this page. Nothing here is medical advice.

Go deeper

The reading behind this page

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