There are different methods to calculate safe days to have sex, but none of them can guarantee 100% effectiveness in preventing pregnancy. Here are some methods and information on how to calculate safe days:

1. Calendar Method: This method involves tracking the menstrual cycle to predict the fertile window. To use this method, find the shortest cycle in your past record, subtract 18 from the total number of days in that cycle, count that number from day 1 of your current cycle, and mark that day with an X. The day marked X is your first fertile day. To predict the last fertile day in your current cycle, find the longest cycle in your record, subtract 11 from the total number of days in that cycle, count that number from day 1 of your current cycle, and mark that day with an X. The day marked X is your last fertile day.

2. Ovulation Method: This method involves tracking ovulation to predict the fertile window. Ovulation usually occurs around day 14 of the menstrual cycle, but it can vary. To use this method, track ovulation by monitoring changes in cervical mucus, basal body temperature, or using ovulation predictor kits. Avoid having sex without a condom or another barrier method during the entire fertile window.

3. Safe Period Calculator: This method involves using a calculator to determine the safe days based on the length of the menstrual cycle. The safe period is calculated by subtracting 18 from the shortest cycle and 11 from the longest cycle. The days before and during menstruation are the least fertile days of the menstrual cycle. However, this method is not completely reliable since intercourse itself may trigger untimely ovulation.

Its important to note that none of these methods can guarantee 100% effectiveness in preventing pregnancy. Using birth control, condoms, or another barrier method is typically the best way to prevent pregnancy.