There are 26 biweekly pay periods in a year. Bi-weekly means occurring every second week. Although most work weeks are only 5 days, pay periods operate on 7-day rotations. There are 52 weeks in a year, which divided by 2 equals 26.
Although 26 biweekly pay periods fit almost perfectly into the measure of a calendar year, the first pay cycle of each year does not necessarily begin on January 1st. Therefore, there is typically either one extra or one fewer biweekly pay period in each calendar year, depending on the dates on which the pay periods begin and end.