How does the bbt chart reader work?
- After ovulation, progesterone raises your resting temperature by about 0.2 to 0.5 °C, and it stays up until your next period.
- We look for three readings in a row that are at least 0.2 °C (0.4 °F) above the highest of the six before them. This is called the "3 over 6" pattern.
- The cover line is drawn just above those six lower readings. Ovulation most likely happened the day before the first high reading.
- Missed days are fine: leave a blank line or type "x". Illness, alcohol, poor sleep and changing your waking time can all throw readings off.
Taking a good reading
Take it as soon as you wake, before getting up, talking or drinking, at about the same time each day, after at least 3 hours' sleep. A basal thermometer that reads to two decimal places helps. Note anything unusual, like illness, alcohol or a late night, because they can push a reading up. More in basal body temperature, explained.
When to see a doctor
- Your charts rarely show a sustained rise and your cycles are irregular.
- Your luteal phase (high temperatures) is often under 10 days.
- You've been trying for a year (or 6 months if 36 or over).
In the UK you can call NHS 111 for advice, any time. In an emergency call 999 (UK) or 911 (US).
Questions people ask
What is basal body temperature?
Your resting temperature, taken first thing in the morning before you get up, ideally at the same time each day with a thermometer that reads to two decimal places.
How much does temperature rise after ovulation?
Usually around 0.2 to 0.5 °C (0.3 to 0.9 °F), sustained until your next period.
Can BBT predict ovulation?
No. The rise comes after ovulation, so it confirms rather than predicts. LH tests and cervical mucus can give earlier signs.
My chart has no clear shift. Did I not ovulate?
Not necessarily. Charts can be noisy, and some people ovulate without a clear rise. If you often see no shift and your cycles are irregular, mention it to a GP.
Can I use this as birth control?
No. Splash is not contraception. If you're avoiding pregnancy, use a method that is.
Is anything I enter saved or sent?
No. Everything runs in your browser. Nothing is stored or sent, and the page makes no network requests when you use it.
Sources
Written by the Splash team · Sources checked Oct 2026 · Updated
