Ovulation Calculator - Simple calculation of your fertility

Is the ovulation calculator also suitable for contraception?

Required inputs for the calculation of ovulation

The ovulation calculator or fertility calculator requires the following two inputs to calculate the future dates of ovulation and fertile days:

Ovulation: Why do you need the first day of the last period?

Together with your cycle length, the date of your next ovulation and the subsequent ovulation days can be calculated by the ovulation calculator based on the first day of your last menstruation and displayed in the ovulation calendar. This is because ovulation regularly takes place around 14 days before the start of your next period. Therefore, the start of the next period is determined on the basis of the first day of your last period and the length of your cycle. It should be noted that ovulation always takes place 14 days before the next menstrual period.

On the day of ovulation, an unfertilized egg in the fallopian tube can be fertilized by a male sperm cell. The lifespan of the female egg and male sperm, in conjunction with ovulation, determines the fertile days (i.e., the days in which sexual intercourse can lead to fertilization). This is because the sperm can survive for several days in the cervix, and the female egg can be fertilized within 12 to 24 hours of implantation in the fallopian tube.

This means that the first chances of fertilization are already around four days before ovulation, and even on the day after ovulation, there are good chances of achieving fertilization.

Ovulation: What role does the duration of the cycle play?

Using the duration of your cycle (first day of your period up to and including the last day before your next period) and the date of the first day of your last period, the ovulation calculator can determine the date of your next ovulation and display it in the ovulation calendar. Since ovulation always occurs around 14 days before the start of your next period, the ovulation calculator first calculates the date of your next period and then goes back 14 days to determine the date of your ovulation. In this way, the ovulation calculator determines your ovulation dates for the next 12 months.

Ovulation (i.e., the implantation of a female egg in the fallopian tube) enables the male sperm to fertilize the egg and thus become pregnant. The number of “fertile days” is based on the lifespan of the sperm in the cervix, which lasts for several days, and the possibility of fertilization of the female egg can last up to 24 hours after ovulation. Therefore, this means that sexual intercourse can successfully lead to fertilization and thus pregnancy as early as four days before the day of ovulation and also one day after ovulation.

What happens during ovulation?

Ovulation (also called follicle rupture or ovulation) is the regular expulsion of a mature, still unfertilized egg cell from the ovary. It then “jumps” into the fallopian tube, where it can then be fertilized by a sperm (i.e., a male sperm cell). Ovulation usually takes place around 14 days before the next menstruation occurs. With a 28-day cycle, ovulation takes place exactly in the middle of the female cycle. However, if the cycle is longer or shorter, this may not be the case. For example, if the cycle is longer, the time from the previous menstrual period to ovulation is longer than the 14 days from ovulation to the next menstrual period.

On the day of ovulation, the probability of fertilization (pregnancy) is 33 percent—one day before it is 31 percent. The other fertile days are before and after this, which is related to the survival of the sperm and the duration of the fertilization of the egg.

What is the formula for calculating ovulation?

The expected ovulation is determined according to the following formula:

How do you calculate the fertile days?

As described, ovulation, and thus the possibility of fertilization of the egg, always occurs 14 days before the start of the next menstrual period. The day of ovulation is a very fertile day, but the days around ovulation are also fertile days.

The first chances of fertilization already exist four days before ovulation, as the sperm can survive for a few days in the cervix. Therefore, sperm that are still alive on the day of ovulation can fertilize the egg (i.e., fuse with the egg).

The day after ovulation is also considered a fertile day, as the female egg can still be fertilized by a single sperm cell within 12 to 24 hours after ovulation.

When is the best time to take a pregnancy test?

The suitable time to take a pregnancy test is one day after the next period has not started.

How does the ovulation calendar work?

The ovulation calculator displays an individual ovulation calendar as the result. This contains all the calculated dates for your ovulation, the fertile days around it and the dates of your menstruation.

The fertile days are marked in yellow or red in the ovulation calendar. The day of ovulation (i.e., the day with the highest fertility) is red. The first day of the period is marked in grey. It is easy to see that ovulation is always 14 days before the next start of the period.

With the left/right buttons in the ovulation calendar, you can know the fertile days of the next months. Meanwhile, tooltips provide further information about the individual days.

An example of the calculation of ovulation using the ovulation calculator is as follows:

Calculation of Mrs Kanin's next ovulation

According to Mrs Kanin's first day of her last period on 21st April and her cycle length of 28 days, her next period will start 28 days later on 19th May.

As described above, ovulation generally occurs around 14 days before your next menstrual period, regardless of cycle length. Therefore, the date of Mrs Kanin's ovulation is 14 days before 19th May. Finally, the fertility calculator calculates:

Calculation of Mrs Kanin's fertile days

The "fertile days" are the days in which sexual intercourse can possibly lead to pregnancy. Although the female egg can only be fertilized by the male sperm for around 12 to 24 hours after ovulation, sperm survive in the cervix for up to five days. Therefore, the rule of thumb is that fertile days start four days before ovulation and further extend by one day.

Accordingly, the following fertile days in the ovulation calendar are calculated for Mrs Kanin based on her ovulation on 5th May:

During this time, the probability of fertilization is high in purely mathematical terms, with it being highest one day before ovulation and on the day of ovulation itself at 31 and 33 percent, respectively.

Calculating the earliest date for a pregnancy test

With modern tests, pregnancy can be detected before the next menstrual period. However, it is more reliable to wait for the absence of the next menstruation in order to carry out the pregnancy test on the following day. In the ovulation calendar of the ovulation calculator, this is indicated in the tooltip for the next period.

Calculating ovulation and fertile days for the next twelve months

The ovulation calendar of the ovulation calculator also shows the future dates of the next twelve months for ovulation and the respective fertile days. For this purpose, the forward or backward buttons can be selected in the ovulation calendar. The results are all based on the two entries in the calculator.