In order to calculate safe period you should know little bit knowledge of Menstrual cycle and the time period for which ovum and sperms survive.
Life of Ovum is 48 hours after ovulation while life of sperms is four days in female genital tract.
Ovulation is the process in which ova is shed from ovary so you should also know the date when ovulation will take place in your wife.
To Calculate the ovulation this method is preferred
1. Temperature Method: women’s body temperature is recorded every morning, just after awakening, it should not be taken after drinking some tea or brushing teeth and plotted on a graph paper daily, the curve comes in shape of a valley between two mountains, see the graph curve from infertility rocks. The temperature is low during actual menstruation, subsequently it rises, at the middle of cycle there is fall in temperature followed by a rise. This rise indicates ovulation has occurred.
Since sperm life is 4 days and ovum life is 2 days so if person copulates before 4 days to 2 days after ovulation, she will conceive pregnancy and this period is known as Fertile Period.
Rest of the days other than fertile period is known as SAFE PERIOD.
There is one more way to calculate the Safe and Fertile period read below
Menstrual Cycle varies from 26 t0 31 days from female to female. So safe period is calculated as
Longest cycle days minus (-) 10 days gives you last day of fertile period.
Smallest cycle days minus (-) 18 days gives you first day of fertile period.
for example if menstrual cycle of your wife varies from 28 to 30 days
so 30-10= 20 th day is the last day of fertile period and 28-18=10th day is the first day of fertile period. And rest other days are Safe Period.
Watch out the video on youtube below explaining how to calculate safe period to avoid pregnancy.