How to calculate your safe & unsafe periods

Calculating the “safe” or infertile periods during a woman’s menstrual cycle is one of the oldest forms of contraception and family planning. It determines at which point during a cycle a woman is ovulating and so could become pregnant if she engages in penetrative sex. Calculating safe and unsafe periods is often used as a contraceptive method by religious couples whose faith outlaws artificial contraception.


Consider that the below calculation is for those women who have a regular menstrual cycle of between 26 and 31 days. The periods each month must last less than eight days. About 80 percent of women have menstrual cycles that fall into these parameter. Women with abnormal menstrual cycles should consult a medical professional to help them calculate their safe and unsafe periods.

Over time

Make your calculations over six months. This will give your results a greater degree of accuracy. Observe your periods and note the number of days between the start date of one and the next period. Over six months this will enable you to determine the shortest and longest cycles.

First day

Subtract 18 from the days in your shortest cycle. This gives you the first day of your fertile or unsafe period. For the above example the equation would be 26-18=8.

Last day

Subtract 10 from the days in your longest cycle. This determines the last day of your fertile or unsafe period. For a regular cycle the equation would be 31-10=21.

Unsafe period

Note that the results from the two equations determine your unsafe period, during which you can become pregnant. In the above example, the unsafe period runs between the eighth and 21st days of each menstrual cycle.


Remember that sperm can live for up to five days within a woman’s body. If a woman wants to have sexual intercourse during her “unsafe” or fertile period, she should ensure her partner wears a condom. This also helps protect against sexually transmitted diseases. Do not use the “safe” and “unsafe” calculation immediately after giving birth. Wait for at least six full menstrual cycles before trying to use this method of contraception.

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About the Author

Dirk Huds has been a writer/editor for over six years. He has worked for bookshops and publishers in an editorial capacity and written book reviews for a variety of publications. He is currently studying for his master's degree.