Menstrual cycle calculator
Estimate the date of your next ovulation, your fertile window and your next periods from the date of your last period and the usual length of your cycle. This tool provides a statistical estimate, calculated entirely in your browser.
How it works
This calculator uses the calendar method, the most commonly used way to estimate the key dates of a menstrual cycle: it starts from the first day of your last period and projects the following cycles based on the average cycle length you enter.
How is ovulation estimated?
Ovulation usually occurs about 14 days before the start of the next period (not 14 days after the start of the last period, unless your cycle is exactly 28 days long). The tool therefore first calculates the start date of the next cycle, then counts back 14 days to estimate ovulation, before adding a fertile window of a few days around that date.
Is this estimate 100% reliable?
No. The calendar method assumes a perfectly regular cycle, which isn't always the case: stress, illness, travel or natural hormonal variations can shift actual ovulation by several days. This tool is provided for guidance only and should not be used as your sole method of contraception or pregnancy planning; consult a health professional for appropriate follow-up.
This personal and sensitive data is never sent to a server: the calculation runs only on your device.