Took your basal temp at the wrong time? Use this tool to adjust an early or late reading back to your usual check time. It returns the adjusted BBT and the difference.
How the adjustment works
- Linear time correction: morning BBT tends to rise as the day begins. The calculator applies a linear correction of 0.072 °F per hour (≈0.04 °C/h) toward the usual time. Later reading → subtract; earlier reading → add. A cap (default 8 hours) avoids over-extrapolation.
- Circadian rhythm (optional): body temperature follows a daily cycle with a pre-dawn nadir. If enabled, a cosine model adjusts from the actual clock time to the usual time using a default peak-to-trough amplitude of 0.9 °F and nadir at 04:00. You can edit amplitude or nadir.
- Alcohol (optional): evening alcohol can elevate overnight temperature. The tool removes an estimated bias of 0.18 °F per standard drink that decays with a 6-hour half-life.
- Short sleep (optional): insufficient sleep may nudge BBT upward. The tool subtracts 0.18 °F if sleep is under 4 hours, or 0.09 °F if 4–5.9 hours.
- Sum of effects: the final adjusted value equals the measured temperature plus the linear, circadian, alcohol, and sleep adjustments (signs chosen to “move” the reading to your usual time).
Units and inputs
- Default units: °F for U.S. users (toggle °C if preferred). Conversion:
°F = (°C × 9/5) + 32. - Times: enter 24-hour HH:MM. Large gaps wrap across midnight and are limited by the cap.
Assumptions & limitations
- BBT is most reliable immediately on waking after sufficient sleep. Illness, medications, or major schedule shifts can outweigh the small morning drift.
- Rates and add-ons are pragmatic heuristics grounded in thermoregulation literature and fertility-charting practice; individuals vary.
- If using the circadian option, tailor amplitude and nadir to your own data where possible.
- Refinetti G. The circadian rhythm of body temperature. Front Biosci. 2008;13:586–597. PubMed
- Kräuchi K, Deboer T. Interrelationship between sleep regulation and thermoregulation. Front Biosci. 2010;15:604–625. PubMed
- Roehrs T, Roth T. Sleep, sleepiness, and alcohol use. Alcohol Res Health. 2001;25(2):101–109. PubMed
- CDC. Fertility awareness-based methods: BBT guidance. cdc.gov
How to use this calculator
- Enter your usual temperature check time (the time you normally take BBT).
- Enter the actual time you took today’s reading.
- Enter your measured temperature (°F or °C).
- The tool returns the adjusted temperature for your usual time and the difference.
Use on days with typical sleep and no fever, alcohol, or illness. If you were sick or had disrupted sleep, consider discarding the point.
Last updated: September 21, 2025
Frequently asked questions
What does “adjusted temperature” mean?
It’s your measured BBT corrected back to your usual check time using a standard time-adjustment slope. The calculator also shows the difference from the original reading.
How far off can my time be?
Small shifts (for example, within 1–2 hours) are most suitable. Large shifts or fragmented sleep can distort BBT. If your sleep was disrupted, consider discarding the data point.
Should I adjust if I was sick, drank alcohol, or got less sleep?
No. Illness, alcohol, and poor sleep can raise BBT beyond what time correction can fix. Mark the point as questionable or discard it and rely on other signs like OPK timing or cervical mucus.
Does the unit matter (°F vs °C)?
No. The tool converts and adjusts consistently in either unit. You can toggle °F/°C in the input and the result.
How should I use adjusted BBTs in charting?
Use them sparingly and note which points were adjusted. Identify your thermal shift with the overall pattern plus other clues. For a full interpretation, see the BBT Ovulation Calculator.
Can I rely on adjusted BBT to confirm ovulation?
BBT supports ovulation confirmation but should be paired with other data. Combine BBT with OPK, cycle dates (Menstrual Cycle), and symptoms for best accuracy.
What if today’s adjusted value shifts my coverline?
One point rarely changes the overall chart. If an adjusted value unusually alters your coverline, review sleep quality and discard if needed. Focus on the multi-day pattern.
When should I test for pregnancy?
A sustained temperature rise after ovulation can be supportive but is not diagnostic. For timing, see the Pregnancy Test Accuracy Calculator.