There are several online ovulation calculators that can help you determine your most fertile days and increase your chances of getting pregnant. Here are some of the most popular ones:

1. Tommys Ovulation Calculator: This calculator asks you to select the date of the first day of your last period, adjust your average cycle length, and click submit. It then provides you with your upcoming fertile dates over the next 4 months.

2. Conception Calculator: This calculator estimates a range of days during which a woman is most likely to conceive, as well as a corresponding due date based on a womans average menstrual cycle. It suggests that regular sexual intercourse 5 days before as well as on the day of ovulation can increase the probability of successfully conceiving.

3. Office on Womens Health Ovulation Calculator: This calculator estimates the days you are most likely to be fertile and can increase your chance of getting pregnant. It assumes that there are about 6 days during each menstrual cycle when you can get pregnant, which is called your fertile window. You can use the calculator to see which days you are most likely to be fertile.

4. Clearblue Ovulation Calculator: This calculator estimates your potential ovulation days by taking your average cycle length and the date of your last period. It provides an estimate of how likely you are to ovulate on certain days in your cycle and can help you identify your most fertile days. However, it should be used for informational purposes only, and an ovulation test can provide a more accurate prediction for the day you ovulate and the days leading up to ovulation.

Its important to note that these calculators are just a guide to help you understand when you may be most fertile. They cannot guarantee pregnancy or birth control, and actual ovulation will vary for each woman. If you have irregular periods or are experiencing difficulty getting pregnant, its best to consult a doctor or fertility specialist.