Mercer County area

Population and density of Mercer County

The Census Bureau estimates that 15,229 people lived in Mercer County on July 1, 2025. That ranks #2,145 of 3,222 county equivalents nationally and #68 of 102 in Illinois. Since the April 1, 2020 estimates base of 15,701, the population has declined by 472 (−3%), compared with −0.8% for Illinois as a whole.

With 561.2 square miles of land, that is 27 people per square mile (10 per km²), placing Mercer County #2,088 nationally for density and #78 in Illinois. The median county equivalent has 47 people per square mile; the Illinois median is 44. Put another way, there are about 24 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 Mercer County

12 incorporated places lie at least partly in Mercer County. Together they are home to 8,608 people (56.5%); the other 6,621 (43.5%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is Aledo city, with 3,506 residents in the county. By land, Keithsburg city is the largest at 2.558 mi².

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

Metropolitan area

Mercer County is an outlying county of the Davenport-Moline-Rock Island, IA-IL Metropolitan Statistical Area. Its 4 county equivalents have 380,452 residents on 2,270 square miles of land; Mercer County accounts for 24.7% of the land and 4% of the residents. By combined land it ranks #143 of 393 metropolitan areas, and #149 by population. It belongs to the Davenport-Moline, IA-IL Combined Statistical Area.

ZIP codes

18 ZIP Code Tabulation Areas have land in Mercer County; 13 of them have most of their land inside the county, and the others mainly cover neighboring areas.

612316125961260612626126361264612726127361276612796128161412614426146561466614726147661486

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 Mercer County

Mercer County shares a land border with 7 county equivalents, including 2 in Iowa. It has more land than 5 of them. The largest neighbor is Henry County (823.2 mi²) and the smallest is Henderson County (378.8 mi²). Together with its neighbors, Mercer County forms a block of 4,267.3 mi² of land, of which it holds 13.2%.

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

Mercer 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 99 areas the size of Mercer County.

Another way to picture it: the same land area would fill a square about 23.7 miles (38.1 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 Mercer County?

Mercer County has more land than 1,379 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 Mercer County at #1,940 nationally by total area, 97 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 1.3%, compared with a national county-equivalent median of 1.2% — 0.1 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 Mercer 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.