How Cycle predicts your next period

How the estimate works

• A period starts on the first day you mark with flow. Two flow days with one empty day or none between them are one period.
• A cycle is the number of days from the first day of one period to the first day of the next period.
• Cycle takes the average of your last 6 cycles. If you have fewer, it uses all of them. It rounds the average to whole days.
• The estimate is the first day of your last period plus the average cycle.
• The range goes from your shortest to your longest recent cycle, added to the first day of your last period.
• The average period is the mean number of days from the first to the last flow day, for your last 6 periods.
• The calendar shades the days of the next 3 estimated periods. A shaded day is a guess, not a fact.

When there is not enough data

With fewer than two period starts, Cycle cannot measure a cycle. It uses 28 days as a simple default. It tells you that the number is only an estimate and asks you to log more cycles.

What the estimate cannot do

• It is not medical advice and it does not find any condition.
• It is not birth control. Do not use it to avoid pregnancy.
• Cycles change. The next period can start sooner or later than the estimate.
• If you forget to log a period, the next estimate is less exact.
• Ask a doctor or nurse about pain, heavy flow, missed periods or any other health question.

Sources

The NHS says that a cycle of about 21 to 35 days is common. It also says that periods can last 2 to 7 days.

• NHS: Periods (nhs.uk)

Cycle does not use these numbers to judge your cycle. It only uses the days you log. See the privacy page for what happens to your data.