County outline
Where St. Mary Parish is located
Area breakdown
Land accounts for 49.9% of St. Mary Parish’s measured area and water for 50.1%. Its combined area converts to 2,900.1 square kilometers or approximately 716,623 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 558.528 | 1,446.581 | 357,457.9 |
| Water | 561.195 | 1,453.488 | 359,164.8 |
| Total | 1,119.723 | 2,900.069 | 716,622.7 |
How St. Mary Parish compares
By original Census land area, St. Mary Parish ranks #43 of 64 in Louisiana and #1857 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 0.865 times the Louisiana county-equivalent median of 645.9 square miles. It accounts for 1.3% of the combined county land area in the state.
See the complete Louisiana ranking · What counts as a county equivalent?
Population and density of St. Mary Parish
The Census Bureau estimates that 46,552 people lived in St. Mary Parish on July 1, 2025. That ranks #1,077 of 3,222 county equivalents nationally and #23 of 64 in Louisiana. Since the April 1, 2020 estimates base of 49,405, the population has declined by 2,853 (−5.8%), compared with −0.9% for Louisiana as a whole.
With 558.5 square miles of land, that is 83 people per square mile (32 per km²), placing St. Mary Parish #1,122 nationally for density and #24 in Louisiana. The median county equivalent has 47 people per square mile; the Louisiana median is 48. Put another way, there are about 7.7 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 49,149 | 48,239 | 47,900 | 47,236 | 46,973 | 46,552 |
| Change | — | −910 | −339 | −664 | −263 | −421 |
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 Parish
5 incorporated places lie at least partly in St. Mary Parish. Together they are home to 28,677 people (61.6%); the other 17,875 (38.4%) live outside any incorporated place, in unincorporated communities and rural areas.
The largest is Morgan City city, with 10,757 residents in the county (the city extends into 1 other county). The county seat, Franklin city, has 6,224 residents. By land, Franklin city is the largest at 9.962 mi².
| Incorporated place | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Morgan City cityAlso in 1 other county | 10,757 | −6.3% | 5.984 |
| Franklin cityCounty seat | 6,224 | −7% | 9.962 |
| Patterson city | 5,565 | −6.2% | 2.524 |
| Berwick town | 4,455 | −6.6% | 5.901 |
| Baldwin town | 1,676 | −4.4% | 3.217 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside St. Mary Parish. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.
Metropolitan area
St. Mary Parish is a central county of the Morgan City, LA Micropolitan Statistical Area. St. Mary Parish is the area’s only county equivalent, so the micro area has the same 558.5 square miles of land. By combined land it ranks #389 of 542 micropolitan areas, and #254 by population.
ZIP codes
9 ZIP Code Tabulation Areas have land in St. Mary Parish; 7 of them have most of their land inside the county, and the others mainly cover neighboring areas.
703407034270380703927051470522705237053870544
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 Parish
St. Mary Parish shares a land border with 4 county equivalents. It has more land than 1 of them. The largest neighbor is Terrebonne Parish (1,229.9 mi²) and the smallest is Assumption Parish (344 mi²). Together with its neighbors, St. Mary Parish forms a block of 3,444.7 mi² of land, of which it holds 16.2%.
| Bordering county equivalent | Land mi² | Size vs. St. Mary | Compare |
|---|---|---|---|
| Terrebonne Parish | 1,229.9 | 2.2× larger | Compare |
| St. Martin Parish | 737.5 | 1.3× larger | Compare |
| Iberia Parish | 574.7 | about the same | Compare |
| Assumption Parish | 344 | 1.6× smaller | Compare |
How big is St. Mary Parish compared with U.S. states?
St. Mary Parish has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.9 times as much. Louisiana’s county land could hold about 77 areas the size of St. Mary Parish.
Another way to picture it: the same land area would fill a square about 23.6 miles (38 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 Parish?
St. Mary Parish has more land than 1,365 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 Parish at #653 nationally by total area, 1,204 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 50.1%, compared with a national county-equivalent median of 1.2% — 49 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 Parish in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #1856 | Blount County, Tennessee | 558.8 | Compare |
| #1857 | St. Mary Parish, Louisiana | 558.5 | This county |
| #1858 | Mahnomen County, Minnesota | 557.9 | Compare |
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.
Representative coordinates
The Census internal point is 29.629349, -91.463804. It represents a location inside the geography; it is not necessarily the centroid or the address of a courthouse.
Source and precision
U.S. Census Bureau, 2026 Gazetteer Files, Counties. Original ALAND: 1,446,581,792 m²; AWATER: 1,453,489,227 m². Rankings use ALAND; exact ties use ascending GEOID. Tables preserve Census-published square miles to three decimals, with conversions based on those values.
Common questions
How big is St. Mary Parish?
St. Mary Parish, Louisiana has 558.5 square miles (1,446.6 km²) of land and 561.2 square miles of water, for a total area of 1,119.7 square miles. It ranks #1857 of 3,222 U.S. county equivalents by land area.
Is St. Mary Parish the largest county in Louisiana?
No. St. Mary Parish ranks #43 of 64 in Louisiana by land area. The largest is Vernon Parish, with 1,326.7 square miles of land.
What counties border St. Mary Parish?
St. Mary Parish shares a land border with St. Martin Parish, Iberia Parish, Terrebonne Parish, and Assumption Parish.
What is the population of St. Mary Parish?
The Census Bureau estimates St. Mary Parish had 46,552 residents on July 1, 2025, ranking #1,077 of 3,222 county equivalents. That is 83 people per square mile of land.
What is the county seat of St. Mary Parish?
The county seat of St. Mary Parish is Franklin.
What cities and towns are in St. Mary Parish?
St. Mary Parish has 5 incorporated places. The most populous are Morgan City city, Franklin city, Patterson city, Berwick town, and Baldwin town; 17,875 residents live outside any incorporated place.
Is St. Mary Parish part of a metropolitan area?
Yes. It is a central county of the Morgan City, LA Micropolitan Statistical Area.
How many ZIP codes are in St. Mary Parish?
9 ZIP Code Tabulation Areas have land in St. Mary Parish, and 7 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does St. Mary Parish compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 1.9 times as much land.
How many acres of land are in St. Mary Parish?
The land measurement converts to approximately 357,458 acres. Including water, total area is approximately 716,623 acres. One square mile equals 640 acres.
Why might another source show a different area?
Check whether it reports land or total area, its Census vintage, and its rounding. Our guide explains how to reconcile county area figures.
Can this establish a property boundary?
No. Statistical measurements and generalized maps do not establish property lines. Consult the relevant local surveying or land-records authority.