240 Area Code – Rockville, Maryland: Location, Time Zone & History

Maryland's telephone map traces back to 1947, when the original area code 301 was assigned to cover the entire state. That single code lasted decades before growth forced a split: in November 1991, 410 was carved out to serve Baltimore and the Eastern Shore, leaving 301 to cover the western and central part of the state, including the Washington, D.C. suburbs. As demand for phone numbers kept climbing in this densely populated corridor, planners introduced 240 on June 1, 1997, as an overlay sharing the same territory as 301 rather than a geographic split -- Maryland's first overlay arrangement.

Overlay growth didn't stop there. By 2023, a study had projected that available central-office codes across the 301/240 area would run out within months, prompting the addition of a further overlay, 227, on June 14, 2023. Together, the three codes now serve a combined population of roughly 3.8 million people across the shared numbering plan area, based on 2020 Census figures -- a reflection of decades of suburban and exurban expansion around the nation's capital.

Beyond Rockville, the 240 territory reaches cities such as Bethesda, Silver Spring, Frederick, and Hagerstown, communities that grew alongside the region's shifting economic base. Key sectors include:

• Federal government and federal contracting
• Biotechnology and life sciences along the I-270 corridor
• Higher education and research institutions
• Agriculture, tourism, and logistics/manufacturing