An ovulation calculator is used generally by two different types of women: those attempting to achieve pregnancy, and those attempting to avoid it. The ovulation calculator works very much in the same way for both women, however. It maps the most likely times when you are able to become pregnant so that you know when to, or when not to have sexual intercourse.
In order to make an effective ovulation calculator, the best way to start is to purchase an ovulation prediction kit. This test will tell you when you are most likely about to ovulate.
The next step in creating your ovulation calculator is to make a graph. Down the shorter side of a piece of paper (you may find that graph paper is easiest to use, though any paper will do) label down the side of the paper: Cycle Days, Date of the Month, Sexual Contact, Menstruation.
Using this method, the first day of menstruation indicates day 1 of your cycle.
On your graph, in the Cycle Days row, you will start numbering each day. If your average amount of time between cycles is usually monthly, write the numbers up to 40. If longer, write more numbers. In the Date of the Month row, write the number of whatever the date is. (For example, if the first day of your period is the 27th, then the first number in that row should be "27"). For each time that you have sexual intercourse, you should also mark that off with an "X" in the appropriate row underneath the appropriate date. You will also mark every day of menstruation with a "P".
If your ovulation usually occurs along with pelvic pain, mark this also, as it may be a sign of ovulation. This sign of ovulation is the best time to use the ovulation prediction kit, as the results will be more accurate, and you will spend less money on kits just using them at random intervals.
The next step is to subtract 16 from the number of days of your cycle to determine when to use the ovulation kit. For example, if your cycle is 21 days long, then you'll want to start the kit on day 5 of your cycle. If your ovulation is 40 days, then the best time to start the kit is on the 24th day.
Using this method, if trying to obtain pregnancy, the ovulation calculator is fairly reliable.