Basal body temperature (BBT) charting has been used for decades to track fertility. While it won't predict ovulation in advance like OPKs, it's the gold standard for confirming that ovulation actually happened—and it costs almost nothing to do.
How BBT Works: The Science
Your basal body temperature is your body's lowest resting temperature, typically measured first thing in the morning before any activity. It's controlled by hormones that shift throughout your cycle:
Before ovulation (Follicular Phase): Estrogen is dominant. Temperatures stay relatively low, typically between 96.0-97.5°F (35.5-36.4°C).
After ovulation (Luteal Phase): Progesterone takes over. This hormone raises your core body temperature by about 0.2-0.5°F (0.1-0.3°C). Temperatures rise to typically 97.0-98.5°F (36.1-36.9°C) and stay elevated until your period.
The key pattern you're looking for: a sustained temperature rise that stays elevated for at least 3 days. This confirms ovulation occurred.
What a Typical BBT Chart Looks Like
(Lower temps)
(Higher temps)
The "coverline" is a horizontal line drawn on your chart to help visualize the temperature shift. It's typically drawn 0.1°F above the highest of the last 6 low temperatures before the rise. When you see 3+ temperatures above this line, ovulation is confirmed.
How to Take Your BBT: Step by Step
Get a BBT Thermometer
Use a thermometer accurate to 0.1°F (one-tenth of a degree). Regular fever thermometers often only show whole or half degrees—not precise enough for BBT.
Take It First Thing
Measure your temperature immediately upon waking, before getting out of bed, talking, drinking, or doing anything. Keep the thermometer on your nightstand.
Same Time Daily
Try to take your temperature at the same time every morning (within 30 minutes). Temperature naturally rises throughout the morning, so consistency matters.
After Adequate Sleep
You need at least 3-4 hours of uninterrupted sleep before taking BBT. If you woke up multiple times, your reading may be unreliable.
Record Immediately
Write down or log your temperature right away. Use a paper chart, a fertility app like Fertility Friend, or the app that came with your thermometer.
Note Disturbances
Record anything that might affect your reading: illness, alcohol the night before, poor sleep, different wake time, etc. This helps you interpret outliers.
Reading Your BBT Chart: What Patterns Mean
"BBT doesn't predict ovulation—it confirms it. Think of it as looking in the rearview mirror: it tells you what already happened, not what's about to happen."
Things That Can Affect Your BBT
Many factors can throw off your temperature readings. When these occur, note them on your chart:
- Alcohol: Drinking the night before often raises morning temperature
- Poor sleep: Waking frequently or sleeping poorly affects accuracy
- Different wake time: Waking significantly earlier or later than usual
- Illness/fever: Obviously raises temperature
- Sleep with mouth open: Can cause lower readings (if using oral thermometer)
- Travel/time zones: Disrupts your body's rhythm
- Hormonal medications: Some can affect BBT
- Room temperature extremes: Very hot or cold rooms
When you have a clearly affected reading, you can "discard" it (mark it but don't include it in your pattern analysis). One or two off temperatures won't ruin your chart.
BBT Tracking Options: Traditional vs. Wearable
Traditional BBT Thermometers
Pros: Inexpensive ($10-25), simple, no subscription. Cons: Requires strict same-time wake routine, easy to forget, disrupted by irregular sleep.
Wearable BBT Devices
Pros: Tracks temperature continuously while you sleep, works with irregular schedules, uses algorithms to find your true basal temperature. Cons: More expensive ($150-300), may require subscription for full features.
Combining BBT with Other Methods
BBT works best as part of a comprehensive tracking approach:
BBT + OPKs: OPKs predict ovulation (so you know when to have sex), then BBT confirms it happened (so you know your timing was right). The ideal combination for conception timing.
BBT + Cervical Mucus: CM shows you're approaching fertility; BBT confirms you've passed it. Together they bracket your fertile window.
All three methods: CM tells you fertility is approaching → OPK pinpoints the surge → BBT confirms ovulation occurred. Maximum information for maximum confidence.
Estimate Your Ovulation Day
Calculate when to start watching for your temperature shift based on your cycle length.
Ovulation Calculator →How Long Should You Track BBT?
Give BBT charting at least 2-3 cycles to learn your patterns. The first cycle is learning—you're figuring out the routine and your body's baseline. By cycle 2-3, patterns become clear.
Many women track BBT throughout their TTC journey because:
- It confirms each ovulation occurred
- It shows luteal phase length (important for implantation)
- Sustained high temps indicate possible pregnancy
- It helps identify cycle irregularities to discuss with your doctor
The Bottom Line
BBT charting is a valuable tool for understanding your cycle and confirming ovulation. It's inexpensive (just a thermometer), gives you concrete data, and can reveal important information about your hormonal health.
However, remember its key limitation: BBT looks backward, not forward. It tells you ovulation happened, not when it will happen. For timing intercourse, pair BBT with predictive methods like OPKs or cervical mucus tracking.
Start simple: get a BBT thermometer, take your temperature each morning, and log it. After a few cycles, you'll have valuable insights into your unique cycle patterns.