The Pearl Index calculator is the greatest way to compare and evaluate different types of contraception — drugs and methods that prevent you from getting pregnant. 💊

If you're here to discover the birth control failure rate — you've come to the right place! The article below will define the Pearl Index, talk about birth control effectiveness rates, and discuss the ideal vs. real-life table analysis of contraception. You'll also find the example of Pearl Index calculation!

What is the Pearl Index?

The Pearl Index is a way to evaluate the quality of different contraceptive techniques. We usually describe it as the number of women who got pregnant during 1 year of constant use of a given method, out of a group of 100 women.

In other words, if 100 women tried method "A" for an entire year and 6 of them got pregnant, the birth control index for the "A" method would be equal to 6.

💡 The lower the Pearl Index, the better the birth control.

Getting pregnant wouldn't be possible at all if not for the complicated processes that take place in women's bodies every single month! Maybe it's time to get to know how it all works?

The female fertility, explained step by step:

What is the Pearl Index used for?

We can use the Pearl Index for evaluating different methods of pregnancy prevention. It's also one of the best ways to follow the birth control failure rate. It allows us to compare different contraceptives in the most objective fashion possible.

We distinguish two basic Pearl Index results — the ones recorded in real life, and the ones that are only predicted or tested in ideal conditions. As you can expect, the real-life results are usually quite worse than the ideal ones. 🤷‍♀️

How to calculate the Pearl Index?

We've already defined the Pearl Index — now it's time to compute it!

Take a look at the equation that we used in our Pearl value calculator:

Pearl Index = (No. of pregnancies * 12) * 100 / (No. of women in the study * Study duration)


  • Study duration is given in months.

Pearl Index calculation example

Once we know what Pearl Index is, let's follow one of the calculations of the Pearl value calculator — explained step by step.

Let's imagine we're young, ambitious scientists — we managed to gather 55 patients for our study. Since our budget is quite limited, we only managed to follow the patient's story for 3 months. In the end, 4 of our patients become pregnant.

Let's count!

Pearl Index = (No. of pregnancies * 12) * 100 / (No. of women in the study * Study duration)

Pearl Index = (4 pregnancies * 12) * 100 / (55 women * 3 months)

Pearl Index = 29.1

The calculated Pearl Index is relatively high — our study revealed that this method of contraception is not of good quality. ❌

Birth control accuracy chart

Why is there such a difference between expected and real life Pearl Index? 🤔

The real life index considers all the factors that may lower the efficacy of a given birth control method, as:

  • Vomiting, diarrhea;
  • Bad memory and forgetting;
  • Incorrect placement of a device; and
  • Different sicknesses or health problems.
Pearl Index   
MethodExpected Real-life
Combined pill (progestogen + estrogen, the pill)0.18.0
Progestogen-only pill (mini pill)0.53.0
Intrauterine device (IUD), progestogen 0.20.2
Intrauterine device (IUD), copper0.80.8
Contraceptive patch0.59.0
Vaginal ring0.59.0
Progesterone injections0.34.0
Spermicide (destroys sperm) 625.7
Thermal method920.5
Cervical mucus method320.5
Sympto-thermal method 220.5


How to compare different birth control methods?

We can compare different contraceptives using the birth control index, called the Pearl Index. It describes the number of women who got pregnant in a year, despite using a given birth control method.

The lower the Pearl Index, the better the contraception.

What is the koitus interruptus Pearl Index?

Koitus interruptus (coitus interruputs) is the name of the pull-out method in Latin. The Pearl Index of koitus interruptus is close to 25. So, out of 100 women, who will use this method for a year, 25 will become pregnant.

This is a highly ineffective method that definitely shouldn't be used as birth control.

How can I calculate birth control effectiveness rate?

Calculate the birth control index yourself!

  1. Find out how many women took part in the study.

  2. Calculate the duration of a study.

  3. Discover how many of them got pregnant during the duration of the study.

  4. Use the Pearl index equation:

    Pearl index = (Number of pregnancies * 12) * 100 / (Number of women in the study * Study duration in months)

  5. Yay! You got it. 🎉

What percentage of intercourses result in pregnancy?

The pregnancy ratio depends on the day during the menstrual cycle when the couple decides to have sex. Some sources say that on average 1 in 5 unprotected intercourses may end up in pregnancy, while some others state it's around 10-20%.

Łucja Zaborowska, MD
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