Monroe County area

Population and density of Monroe County

The Census Bureau estimates that 34,840 people lived in Monroe County on July 1, 2025. That ranks #1,347 of 3,222 county equivalents nationally and #40 of 102 in Illinois. Since the April 1, 2020 estimates base of 34,956, the population has declined by 116 (−0.3%), compared with −0.8% for Illinois as a whole.

With 385.3 square miles of land, that is 90 people per square mile (35 per km²), placing Monroe County #1,067 nationally for density and #25 in Illinois. The median county equivalent has 47 people per square mile; the Illinois median is 44. Put another way, there are about 7.1 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 Monroe County

7 incorporated places lie at least partly in Monroe County. Together they are home to 23,605 people (67.8%); the other 11,235 (32.2%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is Waterloo city, with 11,132 residents in the county. By land, Columbia city is the largest at 10.6 mi² across all its counties. Among places with at least 1,000 residents in 2020, Waterloo city grew fastest since then (+1.2%).

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

Metropolitan area

Monroe County is a central county of the St. Louis, MO-IL Metropolitan Statistical Area. Its 15 county equivalents have 2,814,421 residents on 7,863.5 square miles of land; Monroe County accounts for 4.9% of the land and 1.2% of the residents. By combined land it ranks #19 of 393 metropolitan areas, and #23 by population. It belongs to the St. Louis-St. Charles-Farmington, MO-IL Combined Statistical Area.

ZIP codes

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

62236622446224862256622646227762278622796229562298

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 Monroe County

Monroe County shares a land border with 5 county equivalents, including 3 in Missouri. Each of them has more land than Monroe County. The largest neighbor is St. Clair County (657.7 mi²) and the smallest is Ste. Genevieve County (499.2 mi²). Together with its neighbors, Monroe County forms a block of 3,282.1 mi² of land, of which it holds 11.7%.

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

Monroe County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 2.7 times as much. Illinois’s county land could hold about 144 areas the size of Monroe County.

Another way to picture it: the same land area would fill a square about 19.6 miles (31.6 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 Monroe County?

Monroe County has more land than 639 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 Monroe County at #2,607 nationally by total area, 24 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 3.3%, compared with a national county-equivalent median of 1.2% — 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 Monroe 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.