When am I most fertile?

In the three days that end on the day of ovulation, and especially the two days before it. Sex in the five days before ovulation can also lead to a pregnancy, because sperm can survive for several days.

How accurate is an ovulation calculator?

It is a good first guess if your cycles are regular, but it cannot see when you actually ovulate. Ovulation can move by several days from one cycle to the next, even in people with regular periods. Ovulation tests, which detect the hormone surge (LH) that comes a day or so before ovulation, give a more personal answer.

Can I use this calculator as contraception?

No. Calendar calculations alone are not a reliable way to avoid pregnancy, because ovulation can come earlier or later than expected and sperm can survive for days. If you do not want to get pregnant, talk to your doctor, a sexual health clinic or a pharmacist about contraception.

Does ovulation always happen 14 days before a period?

Fourteen days is the average. In most people the time from ovulation to the next period is between about 10 and 16 days, and it stays fairly stable from cycle to cycle. That is why the calculator counts back from your next period rather than forward from the last one.

My cycles are irregular. Can I still use it?

You can, but treat the dates as a rough guide. If your cycle length changes by more than about a week from month to month, ovulation tests are more useful. Very irregular periods are worth mentioning to your doctor, especially if you are trying to get pregnant.

When should we see a doctor about getting pregnant?

The usual advice is to see your doctor if you have not conceived after a year of regular unprotected sex. Go sooner if you are 36 or older, if you know of a possible fertility problem, or if your periods are very irregular or absent.

Ovulation calculator: your fertile days

How the calculator works

A menstrual cycle runs from the first day of one period to the first day of the next. Somewhere in the middle, one of the ovaries releases an egg: this is ovulation. The length of the first half of the cycle varies a lot between people and from month to month, but the second half, from ovulation to the next period, is much more stable at around 14 days.

So the calculator works backwards. It takes the first day of your last period, adds your average cycle length to find your next period, and counts 14 days back from it. That day is your estimated ovulation day.

Your fertile window is the five days before ovulation plus the day of ovulation itself. Sperm can survive in the body for several days, usually up to five, while the egg lives for only about 12 to 24 hours after it is released. Sex during those six days can lead to a pregnancy; the chances are highest in the last three, up to and including ovulation day.

The calculator then repeats the same pattern for your next six cycles, assuming each one has the same length.

A worked example

Say your last period started on 1 March and your cycle is usually 28 days.

Your next period is due 28 days later, on 29 March.
Counting 14 days back from 29 March gives an estimated ovulation on 15 March.
Your fertile window runs from 10 to 15 March, and your best chances are on 13, 14 and 15 March.

With a 32-day cycle, everything moves four days later: the next period on 2 April, ovulation on 19 March and a fertile window from 14 to 19 March. With a 25-day cycle, ovulation comes earlier, on 12 March.

Reading the results

The headline shows your next fertile window, or the one you are in now. Below it you see the estimated ovulation day, your three most fertile days and the date your next period is due.

The table lists six cycles. Each row shows the expected period (using your period length, 5 days if you leave the default), the fertile window, ovulation day and the start of the following period. The row marked "Now" or "Next" is the fertile window that matters today.

If the date you entered is more than one cycle ago, the calculator starts from the cycle you are probably in now, and reminds you that your period was expected. A late period can have many causes, including stress, illness, travel and pregnancy. If there is any chance you are pregnant, a pregnancy test from the first day of the missed period can tell you more.

How to get a more precise answer

A calendar estimate is a starting point. Your body gives more direct signs:

Ovulation tests detect the surge of luteinising hormone (LH) that comes about a day or so before ovulation. They are sold in pharmacies and supermarkets.
Cervical mucus becomes clear, slippery and stretchy, like raw egg white, in the days before ovulation.
Basal body temperature, measured first thing in the morning, rises slightly after ovulation. It confirms that ovulation has happened, so it is more useful for learning your pattern than for timing sex in the current cycle.

If you and your partner are trying for a baby, you do not need to time sex perfectly. Having sex every two to three days throughout the cycle covers the fertile window without the pressure of a schedule.

Limits of this calculator

This tool assumes regular cycles and a 14-day second half. Ovulation can come earlier or later than predicted, can be delayed by illness, stress or breastfeeding, and may not happen at all in some cycles. Hormonal contraception, recent pregnancy and conditions such as polycystic ovary syndrome (PCOS) also change the picture.

It is an estimate for planning, never a method of contraception. If your periods are very irregular, painful or absent, or if you have been trying to conceive for a year (or for less time if you are 36 or older), talk to your doctor.

Ovulation and fertile window calculator