County outline
Where Jefferson County is located
Area breakdown
Land accounts for 97.8% of Jefferson County’s measured area and water for 2.2%. Its combined area converts to 1,512 square kilometers or approximately 373,633 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 571.237 | 1,479.497 | 365,591.7 |
| Water | 12.565 | 32.543 | 8,041.6 |
| Total | 583.802 | 1,512.04 | 373,633.3 |
How Jefferson County compares
By original Census land area, Jefferson County ranks #41 of 102 in Illinois and #1760 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 1.119 times the Illinois county-equivalent median of 510.7 square miles. It accounts for 1% of the combined county land area in the state.
See the complete Illinois ranking · What counts as a county equivalent?
Population and density of Jefferson County
The Census Bureau estimates that 36,147 people lived in Jefferson County on July 1, 2025. That ranks #1,324 of 3,222 county equivalents nationally and #38 of 102 in Illinois. Since the April 1, 2020 estimates base of 37,115, the population has declined by 968 (−2.6%), compared with −0.8% for Illinois as a whole.
With 571.2 square miles of land, that is 63 people per square mile (24 per km²), placing Jefferson County #1,347 nationally for density and #38 in Illinois. The median county equivalent has 47 people per square mile; the Illinois median is 44. Put another way, there are about 10 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 36,998 | 36,904 | 36,458 | 36,306 | 36,112 | 36,147 |
| Change | — | −94 | −446 | −152 | −194 | +35 |
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 Jefferson County
10 incorporated places lie at least partly in Jefferson County. Together they are home to 18,749 people (51.9%); the other 17,398 (48.1%) live outside any incorporated place, in unincorporated communities and rural areas.
The largest is Mount Vernon city, with 14,187 residents in the county.
| Incorporated place | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Mount Vernon cityCounty seat | 14,187 | −2.9% | 14.7 |
| Ina village | 1,612 | −2.2% | 2.406 |
| Woodlawn village | 611 | −2.1% | 0.855 |
| Bluford village | 607 | −2.9% | 1.587 |
| Dix village | 456 | −2.6% | 2.074 |
| Waltonville village | 406 | −2.6% | 1.243 |
| Bonnie village | 365 | −1.9% | 1.275 |
| Belle Rive village | 307 | −2.2% | 1.026 |
| Nason city | 198 | −2% | 0.911 |
| Centralia cityAlso in 3 other counties | 0 | — | 8.2 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside Jefferson County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.
Metropolitan area
Jefferson County is a central county of the Mount Vernon, IL Micropolitan Statistical Area. Jefferson County is the area’s only county equivalent, so the micro area has the same 571.2 square miles of land. By combined land it ranks #376 of 542 micropolitan areas, and #382 by population.
ZIP codes
17 ZIP Code Tabulation Areas have land in Jefferson County; 12 of them have most of their land inside the county, and the others mainly cover neighboring areas.
6280162808628106281462816628306283662846628516286462872628776288362889628936289462898
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 Jefferson County
Jefferson County shares a land border with 6 county equivalents. It has more land than 4 of them. The largest neighbor is Wayne County (713.8 mi²) and the smallest is Franklin County (408.9 mi²). Together with its neighbors, Jefferson County forms a block of 3,705.7 mi² of land, of which it holds 15.4%.
| Bordering county equivalent | Land mi² | Size vs. Jefferson | Compare |
|---|---|---|---|
| Wayne County | 713.8 | 1.2× larger | Compare |
| Marion County | 572.5 | about the same | Compare |
| Washington County | 562.6 | about the same | Compare |
| Perry County | 441.9 | 1.3× smaller | Compare |
| Hamilton County | 434.7 | 1.3× smaller | Compare |
| Franklin County | 408.9 | 1.4× smaller | Compare |
How big is Jefferson County compared with U.S. states?
Jefferson County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.8 times as much. Illinois’s county land could hold about 97 areas the size of Jefferson County.
Another way to picture it: the same land area would fill a square about 23.9 miles (38.5 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 Jefferson County?
Jefferson County has more land than 1,462 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 Jefferson County at #1,810 nationally by total area, 50 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 2.2%, compared with a national county-equivalent median of 1.2% — 1 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 Jefferson County in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #1759 | Ionia County, Michigan | 571.3 | Compare |
| #1760 | Jefferson County, Illinois | 571.2 | This county |
| #1761 | Wilson County, Tennessee | 571.1 | 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 38.301660, -88.924236. 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: 1,479,496,292 m²; AWATER: 32,544,476 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 Jefferson County?
Jefferson County, Illinois has 571.2 square miles (1,479.5 km²) of land and 12.6 square miles of water, for a total area of 583.8 square miles. It ranks #1760 of 3,222 U.S. county equivalents by land area.
Is Jefferson County the largest county in Illinois?
No. Jefferson County ranks #41 of 102 in Illinois by land area. The largest is McLean County, with 1,183.2 square miles of land.
What counties border Jefferson County?
Jefferson County shares a land border with Marion County, Franklin County, Washington County, Wayne County, Hamilton County, and Perry County.
What is the population of Jefferson County?
The Census Bureau estimates Jefferson County had 36,147 residents on July 1, 2025, ranking #1,324 of 3,222 county equivalents. That is 63 people per square mile of land.
What is the county seat of Jefferson County?
The county seat of Jefferson County is Mount Vernon.
What cities and towns are in Jefferson County?
Jefferson County has 10 incorporated places. The most populous five are Mount Vernon city, Ina village, Woodlawn village, Bluford village, and Dix village; 17,398 residents live outside any incorporated place.
Is Jefferson County part of a metropolitan area?
Yes. It is a central county of the Mount Vernon, IL Micropolitan Statistical Area.
How many ZIP codes are in Jefferson County?
17 ZIP Code Tabulation Areas have land in Jefferson County, and 12 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does Jefferson County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 1.8 times as much land.
How many acres of land are in Jefferson County?
The land measurement converts to approximately 365,592 acres. Including water, total area is approximately 373,633 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.