Ovulation is tricky to exactly know. There is an average time but that isn't always going to be the case. It can change from month to month, woman to woman. It can be a moving target. The average cycle is 28 days and with that, ovulation usually takes place on day 14. Half way through. We normally have a period if we haven't become pregnant about 14 days after we have ovulated. Are you asking because you are trying to conceive? The best way to get pregnant is to get a pregnancy app (try MYCYCLES here at Med help (above tool bar under Health Tools) and track your period. Day one of your period is cycle day one. Then on day 10, begin have sex every other day for the next 10 days. This is the best way to catch the egg. But to answer your question, and this is just a guess at best, if you have a 31 day cycle, you most likely ovulate on day 17. (however, sperm lives for up to 5 days . . . so you can get pregnant all 5 days before that too). good luck