What are calendar based methods?

Calendar methodCalendar based contraceptive methods prevent pregnancy by monitoring the fertile periods during the menstrual cycle. To use this method a woman monitors the length of her menstrual cycle for a period of six months to establish how many days her cycle usually lasts for. She then tracks the progress through each future cycle from the first day of menstrual bleeding (e.g. by numbering the days in a calendar). Doing this enables a woman to determine whether or not she is fertile on a given day of her menstrual cycle and avoid sexual intercourse on those days.

How calendar based methods work

Calendar based methods work allow women to avoid sexual intercourse or use alternative contraceptive methods during the fertile stages of their menstrual cycle.

Research has demonstrated that women are fertile for only around six days of their menstrual cycle, and that the period of fertility coincides with ovulation (the release of an egg from the ovaries). When an egg is released from the ovaries, it enters the fallopian tubes, where fertilisation can occur. If the egg is not fertilised, it travels through the tubes and is expelled through the vaginal opening. This process takes around 24 hours, and it is only within this 24 hours period that fertilisation of the egg can occur.

However, sperm can survive in the fallopian tubes for up to five days following ovulation. This means that if a woman has unprotected sex in the 5 days prior to ovulation, there is a chance that viable sperm, that is sperm which are still alive and have the capacity to fertilise an egg, may remain in her fallopian tubes until ovulation occurs. Therefore, the woman is considered fertile for six days, rather than 24 hours.

In the vast majority of menstrual cycles, ovulation occurs in middle of the cycle. For women who have standard length menstrual cycles (cycles between 26-32 days in length) 94% of women will experience ovulation within 4 days of their cycle midpoint. Thus by calculating the mid point of the cycle, women can also work out when they are potentially fertile, and when they are definitely infertile. Sex can then be avoided or an alternative method of contraception used on the fertile days of the cycle.

Who can and can not use calendar based methods?

Standard days method

Calendar methodThe standard days method can be used by any woman who wishes to prevent pregnancy and experiences regular menstrual cycles, between 26-32 days in length.

The standard days method should be used with caution in patients with irregular menstrual cycles (e.g. women approaching menopause).

Calendar rhythm method

The calendar rhythm method is a method in which a woman calculates the fertile and infertile days of her menstrual cycle based on the length of her own cycle (as opposed to using a standard length cycle as is the case with the standard days method). This method can therefore be used by women who have irregular length menstrual cycles (menstrual cycles which are always or occasionally <26 days or >32 days). Women must keep track of the length of their menstrual cycle for at least six months before commencing this method.


Correct use of calendar based methods

Calendar method of contraceptionWomen wishing to use calendar based methods should ensure they are fully informed about correct use, as unless the methods are used correctly, their effectiveness is limited. The correct methods for monitoring fertility according to the standard days and calendar rhythm methods are outlined below, and tools are provided to make calculating the fertile period easier. However women should consult a health professional before commencing use of these methods.

It is also extremely important that women using calendar methods do not have unprotected sex during the fertile period. Women who do not wish to abstain from sex during this period should use another method of contraception, for example a diaphragm, which is often used in conjunction with a spermicide to increase contraceptive effectiveness).

Methods of monitoring fertile and infertile periods

Standard days method

The standard days method involves counting off days as the menstrual cycle progresses. In order to use this method effectively, women should:

  • Consider the first day of menstrual bleeding as day one of the menstrual cycle;
  • Keep track of the days of the menstrual cycle using a calendar or Cyclebeads (a string of beads with different colour codes for fertile and infertile days of the menstrual cycle);
  • Avoid unprotected sexual intercourse between days 8-19 of the menstrual cycle, as this is the period of fertility. Women using the standard days method who wish to have sex in this period should use another method of contraception;
  • The method provides contraceptive protection which is 95% effective between days 1-7 of the menstrual cycle, and from day 20 to the commencement of next period of menstrual bleeding.

Calendar rhythm method

The calendar rhythm method is based on the length of an individual woman’s menstrual cycle. The woman must calculate her fertile and infertile periods based on her own menstrual cycle length. To do this effectively women should:

  • Monitor the length of each menstrual cycle for a period of six months before commencing the method;
  • Continue monitoring the length of each menstrual cycle and always use the most recent six menstrual cycles to calculate the fertile days, as explained below;
  • To calculate the first day in the cycle in which she is fertile, the woman should subtract 18 from the number of days of her shortest menstrual cycle in the six month period. This result is the first day in her menstrual cycle where she will be fertile. For example, if a woman’s shortest cycle was 24 days in length, she would first become fertile 6 days after the first day of menstrual bleeding;
  • To calculate the time in the cycle at which she ceases to be fertile, the woman should subtract 11 from the number of days of her longest menstrual cycle. The result indicates the day in her menstrual cycle from which she is no longer fertile. For example, if the longest cycle was 35 days, the woman would be infertile from day 26 of the cycle).

Calendar Rhythm Method

The calendar rhythm method is a calendar based contraceptive method which is suitable for women with irregular menstrual cycles, or those which last less than 26 or more than 32 days.

in order to use the calendar rhythm method, a woman must first monitor the length of her menstrual cycle for six months. Then enter the number of days of each of the previous six cycles in the boxes below. The calculator must be updated at each menstrual cycle, and the most recent six cycles should always be used to work out the fertile period of the current menstrual cycle.

Menstrual cycle length month 1
Menstrual cycle length month 2
Menstrual cycle length month 3
Menstrual cycle length month 4
Menstrual cycle length month 5
Menstrual cycle length month 6


Your shortest menstrual cycle is0 days
Your longest menstrual cycle is 0 days
You are fertile from day 0-0 of your next cycle. 

If you do not wish to get pregnant, your should use an alternative method of contraception between days 0 and 0.

Effectiveness of calendar based fertility awareness methods

The standard days method is 95% effective. In practice the standard days method is only around 88% effective, as couples using the method do not always abstain from sex during fertile periods. However, as some 85% of sexually active women who do not use any form of contraception will become pregnant within one year, the method still provides considerably higher protection than no contraceptive use.

The calendar rhythm method is 91% effective when used according to the guidelines outlined above.

Benefits of calendar based methods

Calendar method of contraceptionThe benefits of calendar based methods include:

  • They are natural and do not cause side effects;
  • They do not require any special devices or procedures and do not cost anything;
  • Women become more knowledgeable about their menstrual cycle when they use the method.

Limitations of calendar based methods

The limitations of calendar based methods include:

  • Women must monitor the length of their menstrual cycle in order to use them effectively;
  • The standard days method is only suitable for women who have regular menstrual cycles;
  • Constant menstrual cycle length monitoring required for the calendar rhythm method may be difficult for some women;
  • Additional contraceptive methods or abstinence are required for a significant portion of the menstrual cycle;
  • While the methods are theoretically highly effective (91-95%) in practice the standard days method is 88% effective and the calendar rhythm method is 86% effective;
  • They do not protect against sexually transmitted infections (STIs). Women who have sexual partners of unknown STI status should be advised to used male or female condoms to protect against STIs.


More information 

Contraception For more information on different types of contraception, male and female anatomy and related health issues, see Contraception.


  1. World Health Organisation. Family Planning: A global handbook for providers. 2007. [cited 2009, June 20] Available from:  www.who.int/entity/reproductivehealth/publications/family_planning/en/
  2. Germano, E. Jenning, V. New approaches to fertility awareness based methods: incorporating the standard days and two days methods into practice. J Midwifery Women’s Health. 2006;51:471-7.

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