Numerous websites offer a free online fertility calculator to determine 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 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.