St. Mary's County area

Population and density of St. Mary's County

The Census Bureau estimates that 116,692 people lived in St. Mary's County on July 1, 2025. That ranks #557 of 3,222 county equivalents nationally and #12 of 24 in Maryland. Since the April 1, 2020 estimates base of 113,772, the population has grown by 2,920 (+2.6%), compared with +1.4% for Maryland as a whole.

With 358.6 square miles of land, that is 325 people per square mile (126 per km²), placing St. Mary's County #476 nationally for density and #13 in Maryland. The median county equivalent has 47 people per square mile; the Maryland median is 335. Put another way, there are about 2 acres of land per resident.

U.S. Census Bureau, Vintage 2025 population estimates. Estimates are revised each year and are not decennial census counts. Density divides the whole population by the whole county’s land area, so it describes an average, not how settlement is distributed within the county. Population sources and methods.

Cities and towns in St. Mary's County

1 incorporated place lies at least partly in St. Mary's County. Together they are home to 5,033 people (4.3%); the other 111,659 (95.7%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is Leonardtown town, with 5,033 residents in the county.

Metropolitan area

St. Mary's County is a central county of the Lexington Park, MD Metropolitan Statistical Area. Its 2 county equivalents have 211,176 residents on 571.8 square miles of land; St. Mary's County accounts for 62.7% of the land and 55.3% of the residents. By combined land it ranks #345 of 393 metropolitan areas, and #227 by population. It belongs to the Washington-Baltimore-Arlington, DC-MD-VA-WV-PA Combined Statistical Area.

ZIP codes

27 ZIP Code Tabulation Areas have land in St. Mary's County; 26 of them have most of their land inside the county, and the others mainly cover neighboring areas.

206062060920618206192062020621206222062420626206282063020634206362065020653206562065920660206672067020674206802068420686206872069020692

ZIP Code Tabulation Areas are the Census Bureau’s approximations of USPS ZIP Code service areas, from the 2020 Census relationship files. Lighter codes have less than half of their land in this county. Post-office-box and single-address ZIP Codes have no tabulation area.

Counties bordering St. Mary's County

St. Mary's County shares a land border with 2 county equivalents. It has more land than 1 of them. The largest neighbor is Charles County (457.8 mi²) and the smallest is Calvert County (213.2 mi²). Together with its neighbors, St. Mary's County forms a block of 1,029.6 mi² of land, of which it holds 34.8%.

How big is St. Mary's County compared with U.S. states?

St. Mary's County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 2.9 times as much. Maryland’s county land could hold about 27 areas the size of St. Mary's County.

Another way to picture it: the same land area would fill a square about 18.9 miles (30.5 km) on each side. Real county shapes are rarely square, so this is a scale reference rather than a width or driving distance.

State figures are the sums of each state’s county-equivalent land measurements in the same Census release, so states and counties are compared on the same basis. Published state totals can differ slightly because of rounding. Compare all states · Counties bigger than entire states.

What stands out about St. Mary's County?

St. Mary's County has more land than 562 of the other 3,221 county equivalents in the dataset.

A median is the middle county-equivalent measurement, not the average of the largest and smallest. All three bars use the same linear scale. Very small areas may appear as a thin sliver; the labels retain their values.

Does including water change its rank?

Using land plus water places St. Mary's County at #1,256 nationally by total area, 1,404 places closer to the top than its land rank. The water assigned to this geography moves it ahead of some county equivalents with more land.

Its water share is 53.1%, compared with a national county-equivalent median of 1.2% — 51.9 percentage points higher. This describes the statistical geography’s land/water mix, not flood exposure or developable land.

Closest positions in the national land ranking

These entries sit immediately beside St. Mary's County in the national ranking. They can be in different states and are not necessarily geographic neighbors.

Calculated from the same Census release as the area table. Total-area order uses original ALAND + AWATER; water shares and chart labels use published square miles. Calculation details.