Ovulation calculator

How it works

Ovulation usually happens about 14 days before the next period. The fertile window covers the days most likely for conception: from 5 days before ovulation (sperm can survive up to 5 days) to the day after.

Please note

This is an estimate based on a regular cycle: the actual timing of ovulation can vary month to month. It is not a reliable contraceptive method. To plan or avoid a pregnancy, or if your cycles are irregular, talk to your doctor or gynecologist.

The two halves of the cycle are not alike

The second half of the cycle, from ovulation to the next period, is the stable part: it almost always lasts between twelve and sixteen days, because that is the lifespan of the corpus luteum, and it varies little from woman to woman. The first half is the elastic one: it can last ten days or twenty-five.

That is why the calculation is done backwards, subtracting fourteen days from the expected period, and not forwards from day one. Someone with a 32-day cycle looking for ovulation on day fourteen is looking at the wrong window by almost a week.

Why the window opens five days early

The egg lives a short time, twelve to twenty-four hours. If fertilisation depended on it alone, the fertile window would be one day long. It is sperm that widens it: in the cervical mucus of the fertile days they survive up to five days, so intercourse on Monday can fertilise an egg released on Friday.

For the same reason, when trying to conceive the days that count most are the two or three before ovulation, not the one after: by the time the egg arrives, the sperm need to be there already waiting for it.

Why this is not contraception

A calendar does not predict ovulation: it infers it from a cycle assumed to be regular. But ovulation shifts with stress, fever, travel, short nights or a change of season, and nobody notices until the period arrives — that is, two weeks later. Ovulation four days early moves the fertile window into days the calendar called safe.

If the goal is to avoid pregnancy, this page is not the right tool and no calendar app is either. Fertility-awareness methods, where they work, rest on signs measured every day, not on a forecast: waking basal temperature, cervical mucus, luteinising hormone tests, and they still have to be learned from a professional.

When the arithmetic stops working

If the cycle varies by more than seven or eight days from month to month, if it steadily lasts less than 21 or more than 35 days, or if it skips for months, a calendar forecast loses its meaning: that is not a flaw in the sum, it is that there is no regularity to start from. The same holds right after childbirth, while breastfeeding, after stopping hormonal contraception and in perimenopause. In all those cases the calendar should be replaced by a check-up, not tuned better.

Nearby tools

If pregnancy has already begun, Pregnancy weeks tells you how far along you are and Conception date does the sum in reverse. For the days between two dates there is Difference between two dates.