County outline
Where St. Clair County is located
Area breakdown
Land accounts for 97.6% of St. Clair County’s measured area and water for 2.4%. Its combined area converts to 1,745.4 square kilometers or approximately 431,290 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 657.748 | 1,703.559 | 420,958.7 |
| Water | 16.143 | 41.81 | 10,331.5 |
| Total | 673.891 | 1,745.37 | 431,290.2 |
How St. Clair County compares
By original Census land area, St. Clair County ranks #28 of 102 in Illinois and #1428 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 1.288 times the Illinois county-equivalent median of 510.7 square miles. It accounts for 1.2% of the combined county land area in the state.
See the complete Illinois ranking · What counts as a county equivalent?
What stands out about St. Clair County?
St. Clair County has more land than 1,794 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 #1,510 nationally by total area, 82 places farther from the top than its land rank. Other county equivalents gain more area when their water measurements are included.
Its water share is 2.4%, compared with a national county-equivalent median of 1.2% — 1.2 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 |
|---|---|---|---|
| #1427 | Pulaski County, Kentucky | 658.1 | Compare |
| #1428 | St. Clair County, Illinois | 657.7 | This county |
| #1429 | Allen County, Indiana | 657.1 | 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 38.470198, -89.928546. 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: 1,703,560,251 m²; AWATER: 41,809,675 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 many acres of land are in St. Clair County?
The land measurement converts to approximately 420,959 acres. Including water, total area is approximately 431,290 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.