Rock County is a county in Wisconsin, United States, on the Illinois state line in the southern part of the state. It had a population of 163,687 at the 2020 census and covers 726 square miles (1,880 km2), of which 718 square miles (1,860 km2) is land, centred near 42°40′N 89°04′W.
Janesville is the county seat, and its postal code is 53545. The county has six cities, three villages, a number of unincorporated communities, and twenty towns (civil townships). Rock County is in the Central time zone and is the core of the Janesville-Beloit metropolitan statistical area.
Towns & Villages in Rock County
Rock County has six incorporated cities, three incorporated villages, and a long list of smaller unincorporated communities and crossroads settlements scattered across its twenty towns. In the table below, “CDP” marks a census-designated place with no incorporated government, and “Unincorp.” marks a small named community with no official population count.
| Place | Type | Township | Pop. | ZIP |
|---|---|---|---|---|
| Janesville | City · seat | Janesville | 65,615 | 53545 / 53546 / 53548 |
| Beloit | City | Beloit | 36,888 | 53511 |
| Milton | City | Milton | 5,752 | 53563 |
| Edgerton | City | Fulton | 5,594 | 53534 |
| Evansville | City | Union | 5,559 | 53536 |
| Brodhead | City | Avon | 3,224 | 53520 |
| Clinton | Village | Clinton | 2,143 | 53525 |
| Orfordville | Village | Spring Valley | 1,404 | 53576 |
| Footville | Village | Spring Valley | — | 53537 |
| Afton | Unincorp. | Rock | — | — |
| Anderson | Unincorp. | La Prairie | — | — |
| Avalon | Unincorp. | Spring Valley | — | — |
| Bergen | Unincorp. | Spring Valley | — | — |
| Cooksville | Unincorp. | Union | — | — |
| Emerald Grove | Unincorp. | Harmony | — | — |
| Fulton | Unincorp. | Fulton | — | — |
| Hanover | Unincorp. | Center | — | 53542 |
| Indianford | Unincorp. | Fulton | — | — |
| Johnstown Center | Unincorp. | Johnstown | — | — |
| Lima Center | Unincorp. | Lima | — | — |
| Magnolia | Unincorp. | Magnolia | — | — |
| Milton Junction | Unincorp. | Milton | — | 53563 |
| Newark | Unincorp. | Newark | — | — |
| Newville | Unincorp. | Center | — | — |
| Porters | Unincorp. | Porter | — | — |
| Shopiere | Unincorp. | Turtle | — | — |
| Spring Valley | Unincorp. | Spring Valley | — | — |
| Tiffany | Unincorp. | Center | — | — |
| Turtleville | Unincorp. | Turtle | — | — |
Milton Junction and Milton grew together into one built-up area but remain distinct on the map; Milton Junction sits just north of the city and shares its ZIP code. Johnstown Center is a crossroads inside the Town of Johnstown, not a separate government.
Townships
| Township | Pop. (2020) | Main settlements |
|---|---|---|
| Beloit | — | Surrounds the city of Beloit |
| Janesville | — | Surrounds the city of Janesville |
| Turtle | — | Shopiere, Turtleville |
| Center | — | Hanover, Newville, Tiffany |
| Fulton | — | Edgerton (partly), Indianford |
| Union | — | Evansville, Cooksville |
| Milton | — | Milton, Milton Junction |
| Harmony | — | Emerald Grove |
| Avon | — | Brodhead (partly) |
| Spring Valley | — | Orfordville, Footville, Avalon, Bergen |
| Rock | — | Afton |
| La Prairie | — | Anderson |
| Clinton | — | Surrounds the village of Clinton |
| Johnstown | — | Johnstown Center |
| Lima | — | Lima Center |
| Magnolia | — | Magnolia |
| Newark | — | Newark |
| Porter | — | Porters |
| Bradford | — | Rural, no named village center |
| Plymouth | — | Rural, no named village center |
ZIP Codes
- 53545 — Janesville (west side)
- 53546 — Janesville (east side)
- 53548 — Janesville (downtown/PO boxes)
- 53511 — Beloit
- 53563 — Milton (also serves Milton Junction)
- 53534 — Edgerton
- 53536 — Evansville
- 53520 — Brodhead
- 53525 — Clinton
- 53576 — Orfordville
- 53537 — Footville
- 53542 — Hanover
Small unincorporated communities such as Afton, Anderson, Avalon, Bergen, Cooksville, Emerald Grove, Fulton, Indianford, Johnstown Center, Lima Center, Magnolia, Newark, Newville, Porters, Spring Valley, Tiffany, and Turtleville have no post office of their own and are served by the ZIP code of the nearest city or village.
Geography
Rock County sits in the Rock River valley in the southern tier of Wisconsin, where rolling farmland gives way to the flatter Illinois border country. The Rock River runs north to south through the middle of the county, passing through Janesville and Beloit on its way toward the Mississippi; smaller streams include Turtle Creek and Bass Creek. Of the county’s 726 square miles, about 8 square miles is water.
Rock County borders Dane County to the north, Jefferson County to the northeast, Walworth County to the east, Green County to the west, and Boone County and Winnebago County across the state line in Illinois to the south.
Demographics
The 2020 census counted 163,687 people in Rock County, giving a density of about 228 people per square mile of land. Janesville, the largest community, and Beloit, the second largest, together hold roughly two-thirds of the county’s population. Median household income in the county runs below the Wisconsin median of $74,631, reflecting a mix of manufacturing and agricultural incomes across the smaller towns.
Economy & Local Life
Manufacturing, health care, and agriculture anchor Rock County’s economy. Beloit is home to Beloit College and has drawn advanced manufacturing and logistics investment along the I-90/I-39 corridor; Janesville’s economy has diversified since General Motors closed its assembly plant there in 2008, with Mercy Health System, SHINE Technologies, and Seneca Foods among the larger employers. Farming remains important in the county’s outer towns, with dairy and row-crop operations common. The Rock County 4-H Fair, held each summer in Janesville, is one of the largest county fairs in Wisconsin.
History
Rock County was created as a territorial county on December 7, 1836, carved from Milwaukee County, and was fully organized on February 19, 1839. It takes its name from the Rock River, which the Ho-Chunk and other Native peoples had long used as a travel corridor before white settlement began in the 1830s. Janesville was platted in 1836 and grew as a river-crossing and later a rail and manufacturing town; Beloit developed around Beloit College, founded in 1846, and later became an industrial center on the Illinois line.
Getting There
Interstate 90 and Interstate 39 run together through the county on their way toward Rockford, Illinois, and Interstate 43 crosses the southeast corner near Beloit; U.S. 14 and U.S. 51 also pass through Janesville. Rail service is provided by Canadian Pacific, Union Pacific, and the Wisconsin and Southern Railroad.
- Madison — 25 miles north
- Rockford, Illinois — 15 miles south
- Milwaukee — 65 miles northeast
- Chicago — 100 miles southeast
Southern Wisconsin Regional Airport (KJVL) in Janesville handles general aviation and cargo; the nearest commercial passenger airports are Dane County Regional in Madison and Chicago Rockford International.
Frequently Asked Questions
Where is Rock County, Wisconsin?
It is in the southern part of the state, on the Illinois border, between Madison and Rockford, Illinois.
What is the population of Rock County?
163,687 at the 2020 census.
How big is Rock County?
726 square miles (1,880 km2), of which 718 square miles is land.
What is the county seat of Rock County?
Janesville is the county seat.
How many towns and villages are in Rock County?
Six cities, three villages, and twenty towns (civil townships), plus a number of smaller unincorporated communities.
What ZIP codes are in Rock County?
Major ZIPs include 53545/53546/53548 (Janesville), 53511 (Beloit), 53563 (Milton), 53534 (Edgerton), 53536 (Evansville), 53520 (Brodhead), 53525 (Clinton), 53576 (Orfordville), and 53537 (Footville).
What river runs through Rock County?
The Rock River, which gives the county its name, runs north to south through Janesville and Beloit.
More counties in this state: Towns and villages in Wisconsin.