Area code 314 was one of the original area codes created in October 1947. It originally covered most of the eastern half of Missouri, from the Illinois border to Jefferson City. It stretched along nearly the entire width of the state along the Mississippi River. It was split on January 7, 1996, when most of the western and southern portion was assigned to area code 573, leaving 314 to cover most of the immediate St. Louis metropolitan area.
The creation of 573 was intended to be a long-term solution. Within only two years, however, 314 was close to exhaustion once again due to the proliferation of cell phones, fax machines and pagers. The supply of numbers was further limited because the St. Louis LATA extends across much of the southern portion of Illinois, meaning several numbers that were part of Illinois' 618 weren't available for use. As a result, 314 was reduced to its current size on May 22, 1999, when area code 636 was assigned to St. Louis' outer western suburbs. Before the introduction of number portability, 314 was used for all cell phones on the Missouri side of the St. Louis area.