County outline
Where St. Clair County is located
Area breakdown
Land accounts for 86.3% of St. Clair County’s measured area and water for 13.7%. Its combined area converts to 2,166.5 square kilometers or approximately 535,357 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 721.522 | 1,868.733 | 461,774.1 |
| Water | 114.974 | 297.781 | 73,583.4 |
| Total | 836.496 | 2,166.515 | 535,357.4 |
How St. Clair County compares
By original Census land area, St. Clair County ranks #24 of 83 in Michigan and #1239 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 1.271 times the Michigan county-equivalent median of 567.8 square miles. It accounts for 1.3% of the combined county land area in the state.
See the complete Michigan ranking · What counts as a county equivalent?
Population and density of St. Clair County
The Census Bureau estimates that 160,486 people lived in St. Clair County on July 1, 2025. That ranks #440 of 3,222 county equivalents nationally and #13 of 83 in Michigan. Since the April 1, 2020 estimates base of 160,385, the population has grown by 101 (+0.1%), compared with +0.5% for Michigan as a whole.
With 721.5 square miles of land, that is 222 people per square mile (86 per km²), placing St. Clair County #607 nationally for density and #17 in Michigan. The median county equivalent has 47 people per square mile; the Michigan median is 62. Put another way, there are about 2.9 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 160,386 | 160,268 | 159,886 | 160,279 | 160,304 | 160,486 |
| Change | — | −118 | −382 | +393 | +25 | +182 |
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. Clair County
St. Clair County is divided into 31 municipalities and townships. In Michigan, county subdivisions are local governments, and together they cover all of the county’s land and residents; a few are unorganized areas without their own government.
The most populous is Port Huron city, with 28,273 residents (17.6% of the county). The largest by land is St. Clair township, at 38.4 mi², or 5.3% of the county’s land. Among those with at least 1,000 residents in 2020, Casco township grew fastest since then (+9.8%).
| County subdivision | Population 2025 | Since 2020 | Land mi² | Share of county land |
|---|---|---|---|---|
| Port Huron cityCounty seat | 28,273 | −2.4% | 8.103 | 1.1% |
| Fort Gratiot charter township | 11,715 | +4.2% | 16 | 2.2% |
| Port Huron charter townshipCounty seat | 10,665 | −1.2% | 12.8 | 1.8% |
| Marysville city | 9,868 | −1.3% | 7.328 | 1% |
| Kimball township | 9,652 | +0.5% | 37.1 | 5.1% |
| Clay township | 8,268 | −2.2% | 35.5 | 4.9% |
| St. Clair township | 7,159 | +1% | 38.4 | 5.3% |
| St. Clair city | 5,833 | +6.7% | 2.94 | 0.4% |
| Clyde township | 5,422 | −1.7% | 35.7 | 4.9% |
| Ira township | 5,104 | +2.7% | 17 | 2.4% |
| Casco township | 4,387 | +9.8% | 36.9 | 5.1% |
| Columbus township | 4,303 | +4.7% | 36.6 | 5.1% |
| Mussey township | 4,177 | −1.5% | 35.7 | 4.9% |
| Algonac city | 4,109 | −2.1% | 1.419 | 0.2% |
| Burtchville township | 4,034 | −1% | 15.5 | 2.2% |
Show the other 16 subdivisions
| Name | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Marine City city | 3,997 | −1.9% | 2.156 |
| East China township | 3,661 | −1.2% | 6.595 |
| China township | 3,530 | +0.7% | 34.2 |
| Cottrellville township | 3,469 | +1.7% | 20.9 |
| Riley township | 3,196 | −0.2% | 38.2 |
| Wales township | 3,195 | +0.5% | 37.4 |
| Berlin township | 3,134 | +0.5% | 37.1 |
| Emmett township | 2,515 | 0% | 35.3 |
| Kenockee township | 2,376 | −1.3% | 35.7 |
| Brockway township | 1,880 | −0.7% | 33.7 |
| Yale city | 1,873 | −1.4% | 1.373 |
| Grant township | 1,795 | −1.8% | 29.7 |
| Greenwood township | 1,473 | −1.2% | 35.8 |
| Lynn township | 1,113 | −0.9% | 35.7 |
| Memphis city | 309 | −1% | 0.485 |
| Richmond city | 1 | 0% | 0.247 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each subdivision inside St. Clair County. Land: 2026 Census Gazetteer. How communities are listed.
Metropolitan area
St. Clair County is an outlying county of the Detroit-Warren-Dearborn, MI Metropolitan Statistical Area, in the Warren-Troy-Farmington Hills, MI metropolitan division. Its 6 county equivalents have 4,390,913 residents on 3,892.9 square miles of land; St. Clair County accounts for 18.5% of the land and 3.7% of the residents. By combined land it ranks #75 of 393 metropolitan areas, and #14 by population. It belongs to the Detroit-Warren-Ann Arbor, MI Combined Statistical Area.
| County equivalent | Population 2025 | Land mi² |
|---|---|---|
| Wayne County | 1,769,038 | 611.8 |
| Oakland County | 1,288,337 | 867.4 |
| Macomb County | 886,221 | 479.4 |
| Livingston County | 197,315 | 565.3 |
| St. Clair County | 160,486 | 721.5 |
| Lapeer County | 89,516 | 647.5 |
ZIP codes
23 ZIP Code Tabulation Areas have land in St. Clair County; 21 of them have most of their land inside the county, and the others mainly cover neighboring areas.
4800148002480064801448022480234802748028480324803948040480414804948054480594806048063480644807448079480974841648444
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. Clair County
St. Clair County shares a land border with 3 county equivalents. It has more land than 2 of them. The largest neighbor is Sanilac County (962.6 mi²) and the smallest is Macomb County (479.4 mi²). Together with its neighbors, St. Clair County forms a block of 2,810.9 mi² of land, of which it holds 25.7%.
| Bordering county equivalent | Land mi² | Size vs. St. Clair | Compare |
|---|---|---|---|
| Sanilac County | 962.6 | 1.3× larger | Compare |
| Lapeer County | 647.5 | 1.1× smaller | Compare |
| Macomb County | 479.4 | 1.5× smaller | Compare |
How big is St. Clair County compared with U.S. states?
St. Clair County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.4 times as much. Michigan’s county land could hold about 78 areas the size of St. Clair County.
Another way to picture it: the same land area would fill a square about 26.9 miles (43.2 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. Clair County?
St. Clair County has more land than 1,983 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. Clair County at #1,105 nationally by total area, 134 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 13.7%, compared with a national county-equivalent median of 1.2% — 12.6 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. Clair County in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #1238 | Latimer County, Oklahoma | 722.1 | Compare |
| #1239 | St. Clair County, Michigan | 721.5 | This county |
| #1240 | Tama County, Iowa | 721 | 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 42.928803, -82.668913. 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: 1,868,733,004 m²; AWATER: 297,782,454 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. Clair County?
St. Clair County, Michigan has 721.5 square miles (1,868.7 km²) of land and 115 square miles of water, for a total area of 836.5 square miles. It ranks #1239 of 3,222 U.S. county equivalents by land area.
Is St. Clair County the largest county in Michigan?
No. St. Clair County ranks #24 of 83 in Michigan by land area. The largest is Marquette County, with 1,809.1 square miles of land.
What counties border St. Clair County?
St. Clair County shares a land border with Macomb County, Sanilac County, and Lapeer County.
What is the population of St. Clair County?
The Census Bureau estimates St. Clair County had 160,486 residents on July 1, 2025, ranking #440 of 3,222 county equivalents. That is 222 people per square mile of land.
What is the county seat of St. Clair County?
The county seat of St. Clair County is Port Huron.
What cities and towns are in St. Clair County?
St. Clair County has 31 county subdivisions (cities, towns and townships). The most populous five are Port Huron city, Fort Gratiot charter township, Port Huron charter township, Marysville city, and Kimball township.
Is St. Clair County part of a metropolitan area?
Yes. It is an outlying county of the Detroit-Warren-Dearborn, MI Metropolitan Statistical Area, which has 6 county equivalents and 4,390,913 residents.
How many ZIP codes are in St. Clair County?
23 ZIP Code Tabulation Areas have land in St. Clair County, and 21 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does St. Clair County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 1.4 times as much land.
How many acres of land are in St. Clair County?
The land measurement converts to approximately 461,774 acres. Including water, total area is approximately 535,357 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.