Rock County
Wisconsin · County seat: Janesville · Active county or equivalent
FIPS code 55105 is Rock County, Wisconsin. With a population of 163,817 (2022 ACS estimate), it ranks #9 in Wisconsin and #405 nationally. Rock County is part of the Janesville-Beloit, WI. The state FIPS code for Wisconsin is 55. This code is used in Census Bureau, BLS, EPA, HUD, and USDA federal datasets to identify this county.
County FIPS
55105
Population
163,817
2022 ACS · #405 nationally
Density
228.1/mi²
people per sq mile
Land Area
718
sq miles
Urban/Rural
Metro — under 250K population
RUCC 3
How This FIPS Code Works
The 5-digit FIPS code 55105 is formed by combining the 2-digit state identifier 55 (Wisconsin) with the 3-digit county identifier 105. Together they uniquely identify Rock County in every US federal dataset. No two counties share the same 5-digit FIPS code.
Census ACS Data for FIPS 55105
From the 2022 ACS 5-year estimates — the same values you'd retrieve by querying the Census API with county FIPS 55105.
Median Household Income
$70,630
ACS B19013_001E
Poverty Rate
9.9%
ACS B17001 (below poverty level)
Median Age
40.0
ACS B01002_001E
Metropolitan Area
Adjacent Counties (6)
Counties that share a border with Rock County. Useful for regional analysis — these are the geographies you'd typically include when studying labor markets, commuting zones, or environmental spillovers around FIPS 55105.
Federal Datasets Using Code 55105
FIPS code 55105 appears as the county identifier in the following federal datasets. Analysts joining data across these sources use this code as the common geographic key.
Using FIPS Code 55105 in Code
Common patterns for querying federal datasets by this county's FIPS code. The leading zeros are significant — always treat FIPS codes as strings, not integers.
import requests
url = "https://api.census.gov/data/2022/acs/acs5"
params = {
"get": "NAME,B19013_001E,B01003_001E",
"for": "county:105",
"in": "state:55",
"key": "YOUR_CENSUS_API_KEY",
}
r = requests.get(url, params=params)
data = r.json() # [[header...], [Rock County, Wisconsin, income, pop, ...]-- 5-digit combined FIPS (state + county)
SELECT * FROM your_table WHERE county_fips = '55105';
-- Some datasets store state and county separately
SELECT * FROM your_table
WHERE state_fips = '55'
AND county_fips = '105';
-- ⚠ Never cast to integer — leading zeros are lost:
-- county_fips = 55105 is WRONG for this countylibrary(tidycensus)
get_acs(
geography = "county",
variables = c(income = "B19013_001E", pop = "B01003_001E"),
state = "55",
county = "105",
year = 2022
)ZIP Codes in Rock County
ZIP codes do not align perfectly with county boundaries — a single ZIP code can span multiple counties, and a county can contain parts of many ZIP codes. The ratios below (from the Census 2020 ZCTA relationship file) show the land area share of each ZCTA that falls within Rock County. Why FIPS ≠ ZIP →
| ZIP Code | % in county |
|---|---|
| 53546 | 100.0% |
| 53511 | 100.0% |
| 53536 | 100.0% |
| 53537 | 100.0% |
| 53548 | 100.0% |
| 53545 | 100.0% |
| 53501 | 100.0% |
| 53576 | 100.0% |
| 53505 | 100.0% |
| 53563 | 98.5% |
| 53525 | 94.6% |
| 53520 | 53.3% |
| 53534 | 51.6% |
| 53190 | 27.2% |
| 53114 | 24.5% |
| 53521 | 8.5% |
| 53585 | 4.6% |
| 53589 | 2.1% |
| 53538 | 1.7% |
| 53502 | 1.5% |