Calculator for Ovulation, Fertile Days, and Infertile Days

Calculating ovulation, fertile days, and infertile days

Intelligently Track Fertility and Accurate Ovulation Prediction for Natural Family Planning:

Please note: online fertility calculators should not be used as natural birth control under any circumstances! The results are too uncertain for this purpose. You are using the calculators at your own risk. Any unwanted pregnancy arising from usage of the calculators on this site are under your full responsibility. If you want to be 100% safe, use other contraceptives to avoid pregnancy.

At a Glance

  • Ovulation can’t be determined with 100% accuracy. Sperm can survive 3 to 5 days while the egg itself can survive about 12 to 18 hours. Overall, this results in about 6 fertile days – the day of ovulation and the five days before it.
  • All cycles between 23 and 35 days are considered normal. Longer cycles are more common than short ones. The most common cycle length in women between 20 and 45 years old is 27 days.
  • The myth that the 14th day of the ovulation cycle is the most fertile day is false. The calendar method or Knaus-Ogino method is also considered unreliable. Better methods include the symptothermal method and taking ovulation tests.
  • The most pleasant and effective way of becoming pregnant quickly is regular sex. Regular visits to your gynecologist and observing a healthy, balanced diet helps in improving your chances of getting pregnant.
  • Don’t worry if you can’t become pregnant despite several months of regular sex. Even with a proper diet, the likelihood of getting pregnant within a cycle is only about 20 percent.

Above all else, getting pregnant is a question of proper timing: a woman can only get pregnant around the time of ovulation. The ovulation calculator allows you to identify this time for all cycle lengths. Simply enter the most recent date of menstruation and the fertility calculator will show all fertile and infertile days with an ovulation calendar. But please do not use this calculator alone as a safe period calculator but instead use technical aids as described on the bottom of this page. This is true for regular 28-day cycles as well as longer and shorter ones. Many tips for becoming pregnant and myths about natural family planning are included.

Unfortunately, a lot of misconceptions about periods persist to this day. This dangerous smattering of knowledge is partly responsible for unwanted pregnancies in some young women.

When you're trying to become pregnant, the most important thing is timing. However, it's nearly impossible to calculate the exact time of ovulation.
When you’re trying to become pregnant, the most important thing is timing. However, it’s nearly impossible to calculate the exact time of ovulation.

Conceiving a baby is the dream of most women. But what are good predictors when conception can actually happen? When exactly are you fertile? With this calculator, it’s easy to calculate ovulation, fertile days, and infertile days:

  • Calculate when it’s easiest to become pregnant/conceive a baby (fertile days)
  • Calculate when you can’t become pregnant (infertile days, safe days/safe period)
  • Calculate the timespan in which pregnancy is possible (luteal phase)
  • Calculate when a pregnancy test is possible

How do you recognize fertile days?

First things first: without special tests or examinations, it isn’t possible. Even pregnancy computers that determine the temperature and consistency of cervical mucus can only approximate fertile days.

How does pregnancy work?

Boys to Men Size Conversion

In order to become pregnant, sperm must unite with the egg in the fallopian tube. In this context, the most important date is the ovulation itself. At this point in time, the woman ovulates, which means one or more eggs are released from one of the woman´s ovaries. Here’s the problem: ovulation can’t be determined with 100% accuracy. In a 28-day cycle, for example, ovulation occurs between the 13th and the 17th day. Unfortunately, very few women have cycles of a consistent length.

Sperm can survive 3 to 5 days. The egg itself can survive about 12 to 18 hours. This means that the fertile days in a 28-day sample cycle lie between the 8th day and the 18th day.

On what day does pregnancy become possible?

After ovulating, the luteal phase starts: eggs are fertile for a mere 12 to 18 hours. Sperm, by comparison, live much longer: about 3 to 5 days. Overall, this results in about 6 fertile days (the day of ovulation and the five days before it).

Since the exact time of ovulation can’t be determined precisely, you should anticipate a few safe days if you don’t want to become pregnant.

The cycle

The cycle length is nothing more than the interval between two cycles. The first day of menstruation is the beginning of a new cycle. Unfortunately, only one moment in the cycle is easy to determine: the beginning (because of menstruation).

Normal cycle length

There is no “normal” standard cycle! Every woman has a unique cycle that may also fluctuate from month to month. Additionally, cycle length tends to vary throughout life. Very few women have a consistent cycle length. Irregular cycles are no cause for worry; they are considered normal.

Varying cycle lengths

Generalities about cycles are difficult. For one, fertile and infertile days vary between women. Additionally, cycle lengths vary for each individual woman. For these reasons, it’s better to observe bodily changes to determine fertile days than it is to use calculations. This can be done by a physician or with individual training.

Average cycle length

According to studies, the supposedly common 28-day cycle is seen much less often than expected. Only 12.8% of all cycles last 28 days. Additionally, adult women are more likely to have 28-day cycles than juveniles.

The most common cycle length in women between 20 and 45 years old is 27 days. All cycles between 23 and 35 days are considered medically normal (!). Here’s another fact: longer cycles are more common than short ones. Every 6th cycle lasts 33 days or more. Every 10th cycle lasts 24 days or less. Five percent of cycles in healthy women last more than 35 days.

Calculating fertile and infertile days

Variations in cycle length are completely normal. An individual consultation with a gynecologist makes the most sense for determining the subdivision of fertile and infertile days.


Ovulation usually takes place between the 12th and the 16th day before the next period. During this short period, it is possible to conceive a child.

Menstrual cycles and fertility

The first five days of a period are considered infertile. But this is only true without hormonal influences (the pill!) and if ovulation actually occurred in the previous cycle. From the sixth day of a cycle, fertility is at least theoretically possible.

Beware of traps: myths about fertile days

Unfortunately, there are still textbooks and instructional brochures that spread false information or greatly simplify how things work. For some people, condoms are initially used for contraception. But since they’re perceived as annoying, these people don’t use them at the start or end of a cycle because these are allegedly safe days. Sadly to say, that kind of thinking is mistaken!

The following blanket statements are false

  • Fertility is highest halfway through the standard cycle.
  • The best time for conceiving is exactly halfway between two menstrual periods.
  • Sperm can only survive for 2 to 3 days at most.
  • Pregnancy is impossible during menstruation.
  • Contraception is unnecessary until the 7th day of the cycle.
  • The standard cycle is 28 days long.
  • Ovulation always occurs in the 14th day of the cycle.
  • The days immediately after menstruation (7th to 9th days of the cycle) are still relatively safe.
  • After ovulation, which takes place no later than the 18th day of the cycle, it’s impossible to get pregnant.
  • The week before menstruation (22nd to 28th days of the cycle) is 100% safe.

Warning: this calculation is false

A cycle length of 28 days is often quoted. The (false) conclusion is that ovulation occurs around the 14th day. This is also said to be the most fertile day. If you combine these utter simplifications with the mistaken belief that sperm only lives for 2 to 3 days, you have the perfect formula for chaos. As a result, the false conclusion is that the fertile period – the so-called luteal phase during which conceiving is possible – is between the 11th and the 15th day of the cycle.

Many women know that periods are subject to certain fluctuations. However, they often associate these with the early or late onset of menstruation and not with fertile or infertile days!

Become pregnant quickly: this helps when trying to conceive

The most important thing to know is the exact date of ovulation. With that, the following advice will help make your dream of having children a reality.

The most obvious action is to have sex at the right time. To calculate your fertile days precisely, your cycle has to regain its natural rhythm. The pill and all other hormones should be discontinued. Then give your cycle some time to level off!

Ovulation takes place 12-16 days before the next period. The egg cell can be fertilized 12-18 hours after ovulation. Sperm cells can survive in a woman’s body for up to six days. With this, the fertile period doesn’t just begin with ovulation, but rather six days before it. The calculator above can be used to determine your fertile period.

Be careful: the Knaus-Ogino Method is unreliable

The Calendar Method (sometimes called the Rhythm Method) is named after the doctors Knaus and Ogino. In this method, information about cycles is recorded for at least six months. Afterwards, the fertile days are determined mathematically.

The Knaus-Ogino Method is considered unreliable because it neglects bodily signs (temperature and hormones). Better methods include the Symptothermal Method and ovulation tests.

Symptothermal Method, ovulation tests, pregnancy computers

The Symptothermal Method is markedly more reliable than calculating the date. The temperature and texture of cervical mucus are logged. Alternatively, you can use ovulation tests or a special pregnancy computer. These measure ovulation using daily waking temperature, hormone levels, and/or cervical mucus analysis.

Becoming pregnant quickly

The most pleasant and effective way to become pregnant quickly is regular sex. If you have sex every two days during an entire cycle, with a little luck you can avoid extensive analyses to determine fertile days. However, several methods belong to the realm of fiction. For example, there’s a myth that certain sexual positions like doggy style or missionary give the sperm a shorter route to the egg. Another myth is that supporting the pelvis with a cushion promotes fertilization.

Always a good idea: appointments with a gynecologist and proper nutrition

Do you want to do even more to become pregnant? Visit your gynecologist. He can determine whether the physical conditions necessary for pregnancy are present.

A healthy, balanced diet also paves the way for success. This is true for both you and your partner (bad eating habits affect the quality of sperm, too!). If possible, both of you should avoid coffee, smoking, and alcohol. Foods with calcium, folic acid, zinc, and vitamin C are beneficial (spinach, whole-grain bread, yogurt, omega-3 fats such as those found in salmon, mangos, eggs, milk, pineapples).

How long does it take to become pregnant? What’s normal?

Don’t worry if you can’t become pregnant despite several months of regular sex. Even with a proper diet, the likelihood of getting pregnant within a cycle is only about 20 percent. Stressing out over this makes pregnancy even less likely. Avoid stress. Try to enjoy your life. Exercise. Get some fresh air. Simply relax!

Physicians use the term “inability to conceive” only after two years of unprotected sexual intercourse. Rest assured and consult your gynecologist before you take other measures.

Once you have successfully managed to get pregnant, you can use this calculator to determine the day the baby will be born. You can also check in which trimester the baby is.

Computers and tests: determining ovulation, fertile days, and infertile days

Whether you’re trying to conceive or avoiding pregnancy, there are a number of mini-computers that determine the time of ovulation. With that, it’s easy to determine fertile days. The results can be used either for preventing pregnancy or for planning to conceive. Ultimately, all devices evaluate changes in body temperature or hormone levels measured in urine or saliva.

Please note: cycle computers (hormone computers, temperature computers, or saliva microscopes) are not recommended as the sole method of contraception.

Hormone computers

Devices such as “Persona” (contraception) or “Clearblue” (conception) help you identify fertile days. The devices consist of two parts: single-use test strips and a small handheld monitor. The test strips are read by the handheld computers and a result is shown.

Hormone computers should be used starting on the first morning of menstruation. On every following morning, the computer shows whether it’s a safe day (red light) or a fertile day (green light). A yellow light indicates an unclear status, which means that a urine test should be performed.

Important: The urine test must be performed daily within a certain window of time. This requires a bit of discipline.

Temperature computers

Temperature computers are extremely easy to use. In effect, they are thermometers that also save and interpret previous temperature measurements. After waking up, women must measure their temperatures for about a minute (basal temperature). The device then shows whether it’s a fertile day, a very fertile day, or an infertile day.

The result becomes more accurate with repeated measurements. The advantage of temperature computers is that they can be used with irregular cycles. However, there’s a risk of underestimating the number of infertile days. A more serious disadvantage of temperature computers is that fevers interfere with the results.

Saliva and mucus microscopes

A microscope can be used to test dried cervical mucus or saliva for particular hormones. These show up as crystalline patterns. According to the manufacturer, you should be able to identify them regardless of illness or lifestyle habits. However, it’s difficult for laypeople to interpret the results. For that reason, doctors tend to recommend against this method.

Quick Links