Menstrual cycle calculator
Predict your next periods, ovulation and fertile window from your cycle history.
Menstrual cycle calculator
Paste the start date of your last period and this tool predicts your upcoming periods, ovulation days and fertile windows for as many months ahead as you choose. It works from a single date, or from several past period start dates at once if you want the calculation to use your own average cycle length instead of a guess.
Set the average cycle length and period (bleeding) length to match your own history, and how many months to project forward. Turn on "Derive cycle length from entered dates" and paste two or more past start dates, one per line, to have the tool average the gaps between them instead of using the cycle-length option — useful if your cycle has been drifting and a single textbook number doesn't fit. "Show cycle phases" adds the follicular and luteal phase for each predicted cycle alongside the period, ovulation and fertile window dates. Choose a date format to match how you write dates, which week a calendar week starts on, and whether the result prints as a plain list or as a text calendar with each day marked P (period), O (ovulation) or F (fertile window).
Ovulation is estimated 14 days before each predicted period, and the fertile window as the five days before ovulation plus the day itself — the window during which conception is most likely, not a guarantee. A line that isn't a recognizable date is reported instead of silently skipped, so a typo doesn't quietly disappear from the count.
Everything runs in your browser: dates you paste are never uploaded or stored anywhere. Copy the result, download it as a .txt file, or send it to the input of another tool to keep working with it. This tool gives an estimate based on averages — it is not a method of contraception and shouldn't be relied on to prevent or plan a pregnancy.