How to know when I am ovulating using online calculator?

Not accurate. Online calculators make lots of assumption. First they assume that you ovulate, which is not the case for all women. Then they either assume that your periods are exactly 28 days apart or that you ovulate exactly 14 days before your next period. A better way is to use over the counter ovulation prediction kits which will help you know when exactly you are ovulating and time intercourse .