Numerous websites offer a free online fertility calculator to determine your peak fertility dates. They can be useful but you are probably just as well off keeping your own calendar.
A fertility calculator is based on assumptions about the the ovulation cycle. For women with regular menstrual cycles, ovulation occurs approximately 14 days before the next period; fertility peaks during the 4 days prior to ovulation.
This method of fertility monitoring works best for women who have regular 27-35 day menstrual cycles. More irregular cycles may make it difficult to pinpoint ovulation.
For a more thorough calendar-based solution to fertility monitoring, you might try a fertility calendar product that will be useful not only for monitoring ovulation but for keeping track of other significant dates before and during pregnancy.IVF Success Rate Calculator
IVF Due Date Calculator (FET)