County outline
Where O'Brien County is located
Area breakdown
Land accounts for 100% of O'Brien County’s measured area and water for 0%. Its combined area converts to 1,484.6 square kilometers or approximately 366,853 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 573.035 | 1,484.154 | 366,742.4 |
| Water | 0.173 | 0.448 | 110.7 |
| Total | 573.208 | 1,484.602 | 366,853.1 |
How O'Brien County compares
By original Census land area, O'Brien County ranks #40 of 99 in Iowa and #1744 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 1.006 times the Iowa county-equivalent median of 569.6 square miles. It accounts for 1% of the combined county land area in the state.
See the complete Iowa ranking · What counts as a county equivalent?
Population and density of O'Brien County
The Census Bureau estimates that 14,220 people lived in O'Brien County on July 1, 2025. That ranks #2,215 of 3,222 county equivalents nationally and #56 of 99 in Iowa. Since the April 1, 2020 estimates base of 14,187, the population has grown by 33 (+0.2%), compared with +1.5% for Iowa as a whole.
With 573 square miles of land, that is 25 people per square mile (9.6 per km²), placing O'Brien County #2,150 nationally for density and #53 in Iowa. The median county equivalent has 47 people per square mile; the Iowa median is 26. Put another way, there are about 26 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 14,156 | 14,052 | 14,095 | 14,052 | 14,188 | 14,220 |
| Change | — | −104 | +43 | −43 | +136 | +32 |
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 O'Brien County
8 incorporated places lie at least partly in O'Brien County. Together they are home to 11,106 people (78.1%); the other 3,114 (21.9%) live outside any incorporated place, in unincorporated communities and rural areas.
The largest is Sheldon city, with 5,314 residents in the county (the city extends into 1 other county). The county seat, Primghar city, has 894 residents. Among places with at least 1,000 residents in 2020, Sanborn city grew fastest since then (+5%).
| Incorporated place | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Sheldon cityAlso in 1 other county | 5,314 | +1.3% | 4.828 |
| Hartley city | 1,579 | −1.6% | 1.33 |
| Sanborn city | 1,463 | +5% | 1.878 |
| Paullina city | 970 | −0.9% | 0.934 |
| Primghar cityCounty seat | 894 | −1.3% | 1.359 |
| Sutherland city | 626 | −0.8% | 0.85 |
| Calumet city | 144 | −2% | 0.266 |
| Archer city | 116 | −0.9% | 0.085 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside O'Brien County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.
Metropolitan area
O'Brien 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 O'Brien County; 8 of them have most of their land inside the county, and the others mainly cover neighboring areas.
5100951022510295104651047510585120151231512325123851245512485134651350
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 O'Brien County
O'Brien County shares a land border with 4 county equivalents. It has more land than 2 of them. The largest neighbor is Sioux County (767.9 mi²) and the smallest is Osceola County (398.7 mi²). Together with its neighbors, O'Brien County forms a block of 2,883.8 mi² of land, of which it holds 19.9%.
| Bordering county equivalent | Land mi² | Size vs. O'Brien | Compare |
|---|---|---|---|
| Sioux County | 767.9 | 1.3× larger | Compare |
| Cherokee County | 576.9 | about the same | Compare |
| Clay County | 567.2 | about the same | Compare |
| Osceola County | 398.7 | 1.4× smaller | Compare |
How big is O'Brien County compared with U.S. states?
O'Brien 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. Iowa’s county land could hold about 97 areas the size of O'Brien County.
Another way to picture it: the same land area would fill a square about 23.9 miles (38.5 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 O'Brien County?
O'Brien County has more land than 1,478 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 O'Brien County at #1,897 nationally by total area, 153 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%, 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 O'Brien County in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #1743 | Pierce County, Nebraska | 573.2 | Compare |
| #1744 | O'Brien County, Iowa | 573 | This county |
| #1745 | Polk County, Iowa | 572.7 | Compare |
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.
Representative coordinates
The Census internal point is 43.083746, -95.625624. It represents a location inside the geography; it is not necessarily the centroid or the address of a courthouse.
Source and precision
U.S. Census Bureau, 2026 Gazetteer Files, Counties. Original ALAND: 1,484,153,966 m²; AWATER: 447,448 m². Rankings use ALAND; exact ties use ascending GEOID. Tables preserve Census-published square miles to three decimals, with conversions based on those values.
Common questions
How big is O'Brien County?
O'Brien County, Iowa has 573 square miles (1,484.2 km²) of land and 0.173 square miles of water, for a total area of 573.2 square miles. It ranks #1744 of 3,222 U.S. county equivalents by land area.
Is O'Brien County the largest county in Iowa?
No. O'Brien County ranks #40 of 99 in Iowa by land area. The largest is Kossuth County, with 972.7 square miles of land.
What counties border O'Brien County?
O'Brien County shares a land border with Sioux County, Clay County, Cherokee County, and Osceola County.
What is the population of O'Brien County?
The Census Bureau estimates O'Brien County had 14,220 residents on July 1, 2025, ranking #2,215 of 3,222 county equivalents. That is 25 people per square mile of land.
What is the county seat of O'Brien County?
The county seat of O'Brien County is Primghar.
What cities and towns are in O'Brien County?
O'Brien County has 8 incorporated places. The most populous five are Sheldon city, Hartley city, Sanborn city, Paullina city, and Primghar city; 3,114 residents live outside any incorporated place.
Is O'Brien County part of a metropolitan area?
No. O'Brien County is outside every metropolitan and micropolitan statistical area in the July 2023 OMB delineation.
How many ZIP codes are in O'Brien County?
14 ZIP Code Tabulation Areas have land in O'Brien County, and 8 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does O'Brien County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 1.8 times as much land.
How many acres of land are in O'Brien County?
The land measurement converts to approximately 366,742 acres. Including water, total area is approximately 366,853 acres. One square mile equals 640 acres.
Why might another source show a different area?
Check whether it reports land or total area, its Census vintage, and its rounding. Our guide explains how to reconcile county area figures.
Can this establish a property boundary?
No. Statistical measurements and generalized maps do not establish property lines. Consult the relevant local surveying or land-records authority.