Plymouth County area

Population and density of Plymouth County

The Census Bureau estimates that 25,697 people lived in Plymouth County on July 1, 2025. That ranks #1,644 of 3,222 county equivalents nationally and #25 of 99 in Iowa. Since the April 1, 2020 estimates base of 25,703, the population has declined by 6 (−0%), compared with +1.5% for Iowa as a whole.

With 862.8 square miles of land, that is 30 people per square mile (11 per km²), placing Plymouth County #2,009 nationally for density and #41 in Iowa. The median county equivalent has 47 people per square mile; the Iowa median is 26. 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 Plymouth County

12 incorporated places lie at least partly in Plymouth County. Together they are home to 17,458 people (67.9%); the other 8,239 (32.1%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is Le Mars city, with 10,760 residents in the county. By land, Sioux City city is the largest at 59.4 mi² across all its counties. Among places with at least 1,000 residents in 2020, Le Mars city grew fastest since then (+1.7%).

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

Metropolitan area

Plymouth County is a central county of the Le Mars, IA Micropolitan Statistical Area. Plymouth County is the area’s only county equivalent, so the micro area has the same 862.8 square miles of land. By combined land it ranks #240 of 542 micropolitan areas, and #475 by population. It belongs to the Sioux City-Le Mars, IA-NE-SD Combined Statistical Area.

ZIP codes

15 ZIP Code Tabulation Areas have land in Plymouth County; 11 of them have most of their land inside the county, and the others mainly cover neighboring areas.

510015100851022510245102751028510315103551038510455104851050510625110851109

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 Plymouth County

Plymouth County shares a land border with 4 county equivalents, including 1 in South Dakota. It has more land than 3 of them. The largest neighbor is Woodbury County (872.9 mi²) and the smallest is Union County (462 mi²). Together with its neighbors, Plymouth County forms a block of 3,542.5 mi² of land, of which it holds 24.4%.

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

Plymouth County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.2 times as much. Iowa’s county land could hold about 65 areas the size of Plymouth County.

Another way to picture it: the same land area would fill a square about 29.4 miles (47.3 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 Plymouth County?

Plymouth County has more land than 2,264 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 Plymouth County at #1,061 nationally by total area, 103 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% — 1.1 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 Plymouth 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.