Calendar Method of Family Planning: Calculations, Effectiveness & More

Calendar Method of Family Planning

Medically Reviewed By
Dr. Sukriti Sharma (Gynecologist/Obstetrician)
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Calendar methods of family planning are based on calculating a woman’s fertile period. This calculation is done using recorded data on the length of her previous menstrual cycles. Once the fertile period is calculated, the woman can either abstain from sex on her fertile days or use an alternative method of contraception on those days, such as a diaphragm, spermicide or condom. To know more about the calendar method of contraception, read on.

What Is the Calendar Method of Contraception?

A doctor explaining the menstrual chart to a woman

The calendar method of contraception is a way to prevent unplanned pregnancy by calculating a woman’s fertile period based on the length of her previous menstrual cycles. This data is then used to have sex on the safe days and avoid sex on the fertile days. There are various ways to calculate the calendar method for safe days.

How Does This Method Work?

A woman can get pregnant within a window of around five days in any given menstrual cycle. Once the ovary releases an egg, it moves into the fallopian tube and stays there for 24 hours, waiting to be fertilized by a single sperm. The sperm, on the other hand, can survive for about five days inside the female reproductive tract. After 24 to 48 hours, an unfertilized egg will die and be expelled through the vagina in what is called menstruation.

If you have unprotected sex 2-3 days before you ovulate, the egg may be fertilized by a sperm which has survived inside your reproductive tract, causing you to get pregnant. The calendar method works by predicting the time of a woman’s ovulation so that the woman can abstain from unprotected intercourse during that time to avoid pregnancy.

How to Use the Calendar Method Correctly?

In order to use the calendar method correctly, you need to track the length of your menstrual cycles for at least six months. You mark the first day of your period as Day 1 of the first menstrual cycle. Mark the first day of your next menstrual period. This will be Day 1 of the second menstrual cycle. Count the number of days between the first days of each period. This will be the length of your menstrual cycle. You need to record this for at least six cycles to determine the average length of your menstrual cycle. Using this, you can calculate your fertile period and your safe days. The calendar method works best for women whose menstrual cycles are between 27 to 32 days in length.

Formulas and Calculations Used for Calendar Method

The calendar method has two basic ways of calculating fertile days and safe days in a woman’s menstrual cycle:

1. Standard Days Method

This method was introduced by Georgetown University in 2002. This method had a set of simple rules and was promoted along with a product called the Cycle Beads, a string of coloured beads to help a woman keep track of the high and low fertility days during her menstrual cycle. This method works only for women who have menstrual cycles ranging between 26 to 32 days in length. In this system, days 1-7 of a woman’s menstrual cycle are considered infertile. Days 8-19 are fertile and unsafe to have unprotected sex. The days from day 20 to the end of the cycle are again considered infertile. This method was thought to be 95% efficient in preventing unplanned pregnancies. However, in reality, it is around 88% effective because many women do not abstain and end up having unprotected sex during their fertile days.

2. Calendar Rhythm Method

The rhythm method is best used to determine the best days to have sex if you are planning to conceive. This method is slightly more complex than the standard day’s method. The rhythm method is also called the Knaus-Ogino Method, as it was developed independently by two gynaecologists in the 1920s – Hermann Knaus (an Austrian doctor) and Kyusaku Ogino (a Japanese gynaecologist). In the rhythm method, 19 is subtracted from the length of the woman’s shortest menstrual cycle to find the pre-ovulation infertile period. Then 10 is subtracted from the length of her longest menstrual cycle to find the post-ovulation infertile period. So, a woman whose menstrual cycle ranged from 30 to 36 days will be infertile for the first 11 days of her cycle (30-19=11). She will be fertile on the days between day 12 to 25 and then be infertile again from day 26 (36-10=26). When used to prevent pregnancy, this method was found to be 86 to 91% effective.

Who Can You Use the Calendar Method?

The calendar method to avoid pregnancy works for women whose menstrual cycles range between 27 to 32 days in length. Both the Standard Days method and the Rhythm method do not work for women whose cycles are all shorter than 27 days. The calendar method for pregnancy prevention works for normal, healthy women with regular menstrual cycles. The calendar method will not work for the following:

  • Women who recently gave birth.
  • Women who are breastfeeding.
  • Women are nearing menopause.
  • Women who stopped using hormonal methods of birth control recently.
  • Women who have irregular menstrual cycles.
  • Women whose menstrual cycles are all shorter than 27 days.

Is the Calendar Method for Birth Control Really Effective?

The calendar method of birth control is very effective if used in conjunction with other fertility methods like tracking basal body temperature and checking cervical mucus to predict ovulation. It is also very effective if women abstain from sex during their fertile days or have sex using a backup contraceptive, like condoms, diaphragms or spermicides.

In reality, however, the Standard Days method is only around 88% effective, and the Rhythm method is 86% effective, as women may not always refrain from having unprotected intercourse during their fertile period. The exact time of ovulation is also affected by factors such as illness, medications or stress, making the prediction of safe days inaccurate.

Advantages of Calendar-Based Fertility Awareness Methods

There are several advantages of the calendar-based fertility awareness methods that are used for family planning:

  • They do not cost money.
  • They do not cause side effects like the hormonal methods of contraception do.
  • Women are better aware of their menstrual cycle as they have to monitor it closely to calculate their fertile period. This is helpful in keeping track of their general reproductive health.


There are some disadvantages to using the calendar method to prevent pregnancy:

  • The Standard Days method works only for women who have regular menstrual cycles.
  • The rhythm method will not work for women whose menstrual cycles are shorter than 27 days.
  • It will not work for women who suffer from conditions like polycystic ovaries.
  • The exact timing of ovulation can be affected by stress, medications, and illness. This can make the prediction of the fertile period inaccurate.
  • Additional contraception or abstinence is necessary for the entire fertile period of the menstrual cycle.
  • The actual effectiveness of the Standard Days method is 88%, and that of the rhythm method is 86%, which is a lot less than hormonal or barrier methods of contraception.
  • The calendar method of family planning only prevents pregnancies. It cannot prevent sexually transmitted infections (STIs). Women who have multiple partners or women whose partners are not in a monogamous relationship are at risk of contracting STIs without the use of condoms.

Natural family planning methods such as the calendar method of family planning are not 100% effective in preventing unplanned pregnancies. For them to be more effective, they should be used along with a backup method of contraception such as a cervical cap, spermicide, diaphragm, or condom. The calendar method can also be used by women who are trying to conceive as well. They can have unprotected intercourse during their fertile period to get pregnant. The calendar method requires women to track their menstrual cycle closely to predict their ovulation time. This is now made easier by various apps available online for tracking the menstrual cycle. These apps can even calculate the fertile period and safe days based on previously recorded data. Keep in mind that the calendar method does not prevent STIs, and hence, it is advisable to use condoms for non-monogamous relationships.


1. Rhythm method for natural family planning; Mayo Clinic;

2. What’s the calendar method of FAMs?; Planned Parenthood;

3. Rhythm Method; Cleveland Clinic;

4. Explaining How to Use Calendar-Based Methods; Family Planning Handbook, Chapter 18;

5. Kambic. R. T, Lamprecht. V; Calendar rhythm efficacy: a review; Database of Abstracts of Reviews of Effects (DARE): Quality-assessed Reviews [Internet];; 1996

6. Family planning/contraception methods; World Health Organization;

7. Natural Family Planning or Rhythm Method (Fertility Awareness-Based Method); Alabama Medicaid Agency Plan First/Family Planning Program;; November 2016

Also Read:

Family Planning Tips for Women
Family Planning 101 – Things You Should Invest In
Ways to Calculate Safe Period to Avoid Pregnancy

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