Many of us assume we ovulate on day 14 of our cycles, but this is only the case for women with regular 28-day cycles. What’s more, ovulation doesn’t occur 14 days after the start of menstruation, but 14 days before. The best way to determine when you’re likely to ovulate: select the date you expect your next period to begin and subtract 14 days. Using this formula, a woman with a 24-day cycle will usually ovulate on day 10, while someone with a 30-day cycle can expect ovulation to occur on day 16. Interestingly, women may not be the best judges of their own cycles. An October 2007 study in the American Journal of Epidemiology found that, on average, women overestimated their cycle lengths by nearly a day.