How to Calculate the Rhythm Method

The rhythm method is used to determine a woman's most and least fertile days based on previous menstrual cycles. Used as either an ovulation predictor or a means of contraception, it isn't without its caveats. Women with consistently regular cycles find it most effective, but the majority of women experience menstrual variations throughout their reproductive years. Nevertheless, the rhythm method can be used with other methods of natural family planning to increase their overall rate of effectiveness. Note the start dates of at least six consecutive menstrual cycles before using this method.

Count and note the number of days in each of (at least) the past six months from the start of one menstrual period to the next. Find the months with the shortest fertility cycles (24 days long) and the longest fertility cycles (35 days long). You will use this information for your calculations.

Reduce the number of days in your shortest cycle (24) by 18. This means that you subtract 18 from 24 to get a result of 6. According to the rhythm method, this means that your first fertile day is on the sixth day following the onset of your period.

Use the number of days in your longest cycle to determine the end of peak fertility. Subtract 10 from the number of days in the cycle (35); your answer will be 25. This means that beginning on Day 26 after the start of a menstrual period you will be less likely to conceive.

Interpret the above results to mean that from the sixth to the 25th day in each cycle you should either avoid intercourse (practice abstinence) or use another means of birth control, unless becoming pregnant is your goal. If desired, note the dates of the month that the sixth through the 25th days of your cycle fall on, as a reminder. Use this information as an approximate guide for the upcoming month to predict fertility.

Continue to monitor subsequent cycles, and be alert to any changes in your fertility pattern. Constant vigilance is required to use the rhythm method successfully.


Remember that when used alone as a contraceptive tool, the calendar-based rhythm method is only 80 to 87 per cent effective in preventing pregnancy. This translates to as much as a 20 per cent failure rate. Additional protective measures should be undertaken if becoming pregnant isn't desirable, such as barrier contraceptive methods (condoms, contraceptive spermicide with or without a diaphragm) or pharmaceutical options (birth control pills, injections, IUDs).


Don't assume the rhythm method can provide protection from sexually transmitted diseases. Teens are advised to use other methods of contraception, as rapidly fluctuating hormonal levels prevent their bodies from establishing a consistent pattern of fertility and infertility.

Things You'll Need

  • Calendar
  • Six to 12 months' of data (start dates of previous menstrual cycles)
  • Pencil or pen
  • Notepad (optional)
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About the Author

Genae Valecia Hinesman, former banking executive, entrepreneur and fashion model, began writing professionally in 2002. She is a Cum Laude graduate of the University of Southern California where she studied business, finance and exercise physiology. Her articles featured in Living Healthy: 360, Life 123, the American Chronicle and Yahoo Voices.