Warren County area

Population and density of Warren County

The Census Bureau estimates that 16,222 people lived in Warren County on July 1, 2025. That ranks #2,088 of 3,222 county equivalents nationally and #65 of 102 in Illinois. Since the April 1, 2020 estimates base of 16,844, the population has declined by 622 (−3.7%), compared with −0.8% for Illinois as a whole.

With 542.4 square miles of land, that is 30 people per square mile (12 per km²), placing Warren County #2,006 nationally for density and #72 in Illinois. The median county equivalent has 47 people per square mile; the Illinois median is 44. Put another way, there are about 21 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 Warren County

6 incorporated places lie at least partly in Warren County. Together they are home to 10,788 people (66.5%); the other 5,434 (33.5%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is Monmouth city, with 8,599 residents in the county.

Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside Warren County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.

Metropolitan area

Warren County is not part of any metropolitan or micropolitan statistical area in the Office of Management and Budget’s July 2023 delineation; 1,306 other county equivalents are also outside one. These areas are built around an urban core of at least 10,000 people plus the counties tied to it by commuting, so counties outside them are generally rural.

ZIP codes

14 ZIP Code Tabulation Areas have land in Warren County; 8 of them have most of their land inside the county, and the others mainly cover neighboring areas.

6140161410614126141561417614236143561447614536146261473614746147561478

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 Warren County

Warren County shares a land border with 5 county equivalents. It has more land than 1 of them. The largest neighbor is Fulton County (865.7 mi²) and the smallest is Henderson County (378.8 mi²). Together with its neighbors, Warren County forms a block of 3,653.7 mi² of land, of which it holds 14.8%.

How big is Warren County compared with U.S. states?

Warren County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.9 times as much. Illinois’s county land could hold about 102 areas the size of Warren County.

Another way to picture it: the same land area would fill a square about 23.3 miles (37.5 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 Warren County?

Warren County has more land than 1,300 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 Warren County at #2,029 nationally by total area, 107 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 0.1%, compared with a national county-equivalent median of 1.2% — 1 percentage points lower. 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 Warren 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.