How we calculate your cycle

A new cycle begins on the first day of a period. We detect that as the first bleeding day of a run — a day you marked as bleeding whose previous day was not. A past cycle's length is simply the number of days from one period's first day to the next.

After ovulation, progesterone raises your basal body temperature by roughly 0.5–1.0°F (0.3–0.5°C)¹, and it stays up until your next period. We look for that sustained shift using a well-known “3 over 6” rule from the sympto-thermal method of fertility-awareness charting².

We take the highest temperature of the previous six days as a coverline. The shift is detected on the first day that begins three readings sitting clearly above it. Ovulation is then estimated as the day just before that rise.

It is an estimate from temperature alone, so it can move as you log more mornings. You can confirm or set your ovulation day yourself in the app — a confirmed day always takes priority over the estimate.

Once we know (or estimate) your ovulation day, the rest follows:

• Menstruation — the days you logged bleeding.
• Follicular — after your period and before ovulation.
• Ovulation — the estimated or confirmed ovulation day itself.
• Luteal — from the day after ovulation to your next period.

Before there is enough temperature data to spot a shift, the days after your period are shown as follicular until the rise appears.

We average the gaps between your recent period starts to learn your typical cycle length, then add it to your current period's start date. Cycle length varies between people and shifts gradually with age³, so we learn yours from your own data rather than assuming a textbook 28 days.

A six-day window ending on the day of ovulation is when conception is possible — sperm can survive several days and the egg about a day⁴. Because the luteal phase is more consistent than the follicular phase³, we anchor the estimate on it when ovulation has not been observed yet.

The more you log, the more these reflect your personal pattern rather than textbook averages. You can:

• Confirm or set your ovulation day — it overrides the temperature estimate.
• Set your typical cycle length (21–40) and luteal-phase length (9–18) in Settings.

Cycle Journal starts from population research — the studies above describe what is typical across many people. From there, every entry you log makes the estimates more individualized: your own average cycle length, your luteal length, your temperature shifts, and the ovulation days you confirm all feed back in. We are continually working to derive more personalized, accurate data points from your individual data, so your predictions reflect your body rather than a textbook average.

These remain estimates that improve with more data, and many things — stress, sleep, illness, travel, medication, and life stage — can shift a cycle. They are not a guarantee, and not a substitute for professional care.