Estimate a next period date and calendar-based fertile window from the last period date and average cycle length.
Next Period Prediction
How the Next Period and Fertile Window Are Estimated
The next period date is estimated by adding the entered cycle length to the first day of the last period. The fertile-window estimate is then placed around an assumed ovulation date approximately 14 days before that predicted next period. The calendar uses the dates and average cycle length supplied.
Cycle tracking has important limits
Cycle length and ovulation timing can vary due to stress, illness, medication, hormonal changes, and many other factors. Recording several cycles may help describe a pattern, but it does not make a calendar prediction certain.
Health note: This tool is not a diagnostic tool and must not be used as a birth-control method. Seek medical advice for unusual bleeding, severe pain, missed periods, or fertility concerns.
Example and limitations
Calendar estimates are less reliable when cycles vary, are irregular, or are affected by health conditions, medication, or life stage. They should not be used as contraception or to diagnose a cycle disorder.
Source and important context
The NHS explains that cycle length and ovulation timing vary; a calendar estimate cannot reliably identify a fertile day or be used as contraception. NHS: Fertility in the menstrual cycle. This page provides general educational estimates, not a diagnosis or individualized care advice. Page information updated: 4 November 2026.