County outline
Where Rock County is located
Area breakdown
Land accounts for 98.9% of Rock County’s measured area and water for 1.1%. Its combined area converts to 1,880.7 square kilometers or approximately 464,741 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 718.168 | 1,860.047 | 459,627.5 |
| Water | 7.99 | 20.694 | 5,113.6 |
| Total | 726.158 | 1,880.741 | 464,741.1 |
How Rock County compares
By original Census land area, Rock County ranks #41 of 72 in Wisconsin and #1251 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 0.954 times the Wisconsin county-equivalent median of 753 square miles. It accounts for 1.3% of the combined county land area in the state.
See the complete Wisconsin ranking · What counts as a county equivalent?
Population and density of Rock County
The Census Bureau estimates that 166,472 people lived in Rock County on July 1, 2025. That ranks #426 of 3,222 county equivalents nationally and #9 of 72 in Wisconsin. Since the April 1, 2020 estimates base of 163,684, the population has grown by 2,788 (+1.7%), compared with +1.3% for Wisconsin as a whole.
With 718.2 square miles of land, that is 232 people per square mile (89 per km²), placing Rock County #596 nationally for density and #12 in Wisconsin. The median county equivalent has 47 people per square mile; the Wisconsin median is 51. Put another way, there are about 2.8 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 163,763 | 164,150 | 163,954 | 164,619 | 165,665 | 166,472 |
| Change | — | +387 | −196 | +665 | +1,046 | +807 |
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 Rock County
Rock County is divided into 29 cities and towns. In Wisconsin, county subdivisions are local governments, and together they cover all of the county’s land and residents.
The most populous is Janesville city, with 66,929 residents (40.2% of the county). The largest by land is Newark town, at 36.3 mi², or 5.1% of the county’s land. Among those with at least 1,000 residents in 2020, Orfordville village grew fastest since then (+10.4%).
| County subdivision | Population 2025 | Since 2020 | Land mi² | Share of county land |
|---|---|---|---|---|
| Janesville cityCounty seat | 66,929 | +2% | 34.6 | 4.8% |
| Beloit city | 36,649 | −0.1% | 17.4 | 2.4% |
| Beloit town | 8,014 | +4% | 26.2 | 3.7% |
| Milton city | 6,024 | +5.4% | 4.745 | 0.7% |
| Edgerton city | 5,976 | +3.2% | 3.63 | 0.5% |
| Evansville city | 5,837 | +2.3% | 3.714 | 0.5% |
| Fulton town | 3,715 | +3.8% | 31.4 | 4.4% |
| Janesville townCounty seat | 3,699 | +0.8% | 26.8 | 3.7% |
| Milton town | 3,129 | +0.9% | 29.5 | 4.1% |
| Rock town | 2,988 | +0.6% | 26.2 | 3.6% |
| Harmony town | 2,608 | +2.1% | 21.5 | 3% |
| Turtle town | 2,392 | +0.5% | 28.3 | 3.9% |
| Clinton village | 2,336 | +5.4% | 1.453 | 0.2% |
| Union town | 2,099 | 0% | 32.6 | 4.5% |
| Orfordville village | 1,618 | +10.4% | 1.187 | 0.2% |
Show the other 14 subdivisions
| Name | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Newark town | 1,516 | +0.3% | 36.3 |
| Lima town | 1,279 | +0.9% | 36.2 |
| Plymouth town | 1,175 | −0.3% | 35.5 |
| Center town | 1,044 | 0% | 35.7 |
| Bradford town | 1,002 | −0.8% | 36 |
| Porter town | 988 | +1.2% | 35.9 |
| Footville village | 898 | +7.2% | 1.128 |
| Clinton town | 878 | −1% | 35.2 |
| La Prairie town | 784 | +0.1% | 34.5 |
| Johnstown town | 768 | 0% | 36 |
| Magnolia town | 743 | +0.3% | 35.9 |
| Spring Valley town | 727 | 0% | 34.8 |
| Avon town | 572 | −0.5% | 35.5 |
| Brodhead city | 85 | −2.3% | 0.141 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each subdivision inside Rock County. Land: 2026 Census Gazetteer. How communities are listed.
Metropolitan area
Rock County is a central county of the Janesville-Beloit, WI Metropolitan Statistical Area. Rock County is the area’s only county equivalent, so the metro area has the same 718.2 square miles of land. By combined land it ranks #317 of 393 metropolitan areas, and #270 by population. It belongs to the Madison-Janesville-Beloit, WI Combined Statistical Area.
ZIP codes
20 ZIP Code Tabulation Areas have land in Rock County; 13 of them have most of their land inside the county, and the others mainly cover neighboring areas.
5311453190535015350253505535115352053521535255353453536535375353853545535465354853563535765358553589
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 Rock County
Rock County shares a land border with 6 county equivalents, including 2 in Illinois. It has more land than 5 of them. The largest neighbor is Dane County (1,196.5 mi²) and the smallest is Boone County (280.7 mi²). Together with its neighbors, Rock County forms a block of 4,404.4 mi² of land, of which it holds 16.3%.
| Bordering county equivalent | Land mi² | Size vs. Rock | Compare |
|---|---|---|---|
| Dane County | 1,196.5 | 1.7× larger | Compare |
| Green County | 584 | 1.2× smaller | Compare |
| Jefferson County | 556.5 | 1.3× smaller | Compare |
| Walworth County | 555.4 | 1.3× smaller | Compare |
| Winnebago CountyIllinois | 513 | 1.4× smaller | Compare |
| Boone CountyIllinois | 280.7 | 2.6× smaller | Compare |
How big is Rock County compared with U.S. states?
Rock County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.4 times as much. Wisconsin’s county land could hold about 75 areas the size of Rock County.
Another way to picture it: the same land area would fill a square about 26.8 miles (43.1 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 Rock County?
Rock County has more land than 1,971 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 Rock County at #1,333 nationally by total area, 82 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 1.1%, compared with a national county-equivalent median of 1.2%. The two percentages are within 0.1 percentage points of each other. 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 Rock County in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #1250 | Wichita County, Kansas | 718.6 | Compare |
| #1251 | Rock County, Wisconsin | 718.2 | This county |
| #1252 | Silver Bow County, Montana | 718 | 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 42.669880, -89.075284. 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: 1,860,046,330 m²; AWATER: 20,694,405 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 Rock County?
Rock County, Wisconsin has 718.2 square miles (1,860 km²) of land and 7.99 square miles of water, for a total area of 726.2 square miles. It ranks #1251 of 3,222 U.S. county equivalents by land area.
Is Rock County the largest county in Wisconsin?
No. Rock County ranks #41 of 72 in Wisconsin by land area. The largest is Marathon County, with 1,545.2 square miles of land.
What counties border Rock County?
Rock County shares a land border with Walworth County, Green County, Winnebago County, Illinois, Dane County, Jefferson County, and Boone County, Illinois.
What is the population of Rock County?
The Census Bureau estimates Rock County had 166,472 residents on July 1, 2025, ranking #426 of 3,222 county equivalents. That is 232 people per square mile of land.
What is the county seat of Rock County?
The county seat of Rock County is Janesville.
What cities and towns are in Rock County?
Rock County has 29 county subdivisions (cities, towns and townships). The most populous five are Janesville city, Beloit city, Beloit town, Milton city, and Edgerton city.
Is Rock County part of a metropolitan area?
Yes. It is a central county of the Janesville-Beloit, WI Metropolitan Statistical Area.
How many ZIP codes are in Rock County?
20 ZIP Code Tabulation Areas have land in Rock County, and 13 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does Rock County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 1.4 times as much land.
How many acres of land are in Rock County?
The land measurement converts to approximately 459,628 acres. Including water, total area is approximately 464,741 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.