Maryland's fifth largest city, Rockville, MD is located 12 miles northwest of Washington DC. With a population of more than 53,000 people as of 2005, Rockville has two area codes: 240 and 301.
The 301 area code has covered Rockville since June 1, 1947. Area code 301 was Maryland's only area code until 1991 when its population growth required the addition of a second code, 410. This split the state into 410 in the east and 310 in the west.
On June 1, 1997, area code 240 was added. The two area codes overlay, meaning they cover the same zone. This requires 10-digit dialing for Rockville phone numbers.
Besides 240 and 301, Maryland is also covered by 410 and 443. These two area codes also overlay each other and cover eastern Maryland, including Baltimore. Two additional area codes -- 227 and 667 -- have been proposed but not yet activated, and 227 will provide another overlay for Rockville.
