The Calvert Peninsula is part of the Western Shore region of the U.S. state

U.S. state

A U.S. state is any one of the 50 federated states of the United States of America that share sovereignty with the federal government. Because of this shared sovereignty, an American is a citizen both of the federal entity and of his or her state of domicile. Four states use the official title of...

 of Maryland

Maryland

Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...

. It extends about 25 miles (40.2 km) into Chesapeake Bay with the main bay providing its eastern border and the Patuxent River

Patuxent River

The Patuxent River is a tributary of the Chesapeake Bay in the state of Maryland. There are three main river drainages for central Maryland: the Potomac River to the west passing through Washington D.C., the Patapsco River to the northeast passing through Baltimore, and the Patuxent River between...

 defining its western border. It constitutes a total land area of about 350 square miles (906.5 km²) and is essentially conterminous with Calvert County.

Immediately south of the Calvert Peninsula is the larger St. Mary's Peninsula

St. Mary's Peninsula

St. Mary's Peninsula is part of the Western Shore region of Maryland. From the Charles County line, just north of Charlotte Hall, Maryland, to the southern tip of St. Mary's County at Point Lookout State Park, St. Mary's Peninsula stretches about into the Chesapeake Bay. Its boundaries are drawn...

, defined by the Patuxent and Potomac River

Potomac River

The Potomac River flows into the Chesapeake Bay, located along the mid-Atlantic coast of the United States. The river is approximately long, with a drainage area of about 14,700 square miles...

s.