How is ovulation estimated?
Count the first day of your period as day 1. The calendar method subtracts 14 days from your usual cycle length. In a 28-day cycle, that puts estimated ovulation around day 14 and the estimated fertile window from day 9 to day 14.
What if my cycles are irregular?
Ovulation can shift even in regular cycles. If cycle length varies widely, a calendar estimate is less reliable. A urine LH test can indicate that ovulation is approaching. Do not use this calculator as contraception.
Ovulation test: when to start and how to use it