DeKalb County area

Population and density of DeKalb County

The Census Bureau estimates that 101,835 people lived in DeKalb County on July 1, 2025. That ranks #620 of 3,222 county equivalents nationally and #19 of 102 in Illinois. Since the April 1, 2020 estimates base of 100,522, the population has grown by 1,313 (+1.3%), compared with −0.8% for Illinois as a whole.

With 631.3 square miles of land, that is 161 people per square mile (62 per km²), placing DeKalb County #741 nationally for density and #18 in Illinois. The median county equivalent has 47 people per square mile; the Illinois median is 44. Put another way, there are about 4 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 DeKalb County

14 incorporated places lie at least partly in DeKalb County. Together they are home to 87,782 people (86.2%); the other 14,053 (13.8%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is DeKalb city, with 40,467 residents in the county. The county seat, Sycamore city, has 19,083 residents. Among places with at least 1,000 residents in 2020, Waterman village grew fastest since then (+14.4%).

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

Metropolitan area

DeKalb County is an outlying county of the Chicago-Naperville-Elgin, IL-IN Metropolitan Statistical Area, in the Elgin, IL metropolitan division. Its 13 county equivalents have 9,434,123 residents on 6,923.3 square miles of land; DeKalb County accounts for 9.1% of the land and 1.1% of the residents. By combined land it ranks #27 of 393 metropolitan areas, and #3 by population. It belongs to the Chicago-Naperville, IL-IN-WI Combined Statistical Area.

ZIP codes

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

60111601126011560129601356014060145601466015060151601526017860511605186052060530605316054860550605526055661068

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 DeKalb County

DeKalb County shares a land border with 7 county equivalents. It has more land than 4 of them. The largest neighbor is LaSalle County (1,135.1 mi²) and the smallest is Boone County (280.7 mi²). Together with its neighbors, DeKalb County forms a block of 4,973.6 mi² of land, of which it holds 12.7%.

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

DeKalb County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.6 times as much. Illinois’s county land could hold about 88 areas the size of DeKalb County.

Another way to picture it: the same land area would fill a square about 25.1 miles (40.4 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 DeKalb County?

DeKalb County has more land than 1,702 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 DeKalb County at #1,629 nationally by total area, 109 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 0.5%, compared with a national county-equivalent median of 1.2% — 0.6 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 DeKalb 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.