Massachusetts, United States. It is the principal city of the Pittsfield, Massachusetts Metropolitan Statistical Area which encompasses all of Berkshire County. Its area code is 413. Its ZIP code is 01201 (01202 and 01203 are zip codes for Pittsfield post office boxes only). The population was 44,737 at the 2010 census. Although the population has declined in recent decades, Pittsfield remains the third largest municipality in western Massachusetts, behind only Springfield and Chicopee. - Source: Wikipedia