FIPS Codes vs GEOID: What's the Difference?
If you've worked with Census data, you've probably seen both "FIPS code" and "GEOID" used to identify geographic areas — sometimes for the same place. They're closely related but not identical, and conflating them is a common source of data-join bugs.
The Short Answer
A GEOID is the Census Bureau's own notation for a geographic identifier. For most geography types, a GEOID is simply a concatenation of the relevant FIPS codes, zero-padded to a fixed total length.
King County, Washington
At the county level, the GEOID and the 5-digit county FIPS code are the same thing. The distinction matters when you're working below the county level — with Census tracts, block groups, or blocks — where GEOIDs grow longer by concatenating more FIPS components.
GEOID Length by Geography Type
| Geography | GEOID Length | Formula |
|---|---|---|
| State | 2 digits | State FIPS |
| County | 5 digits | State FIPS (2) + County FIPS (3) |
| Census Tract | 11 digits | State (2) + County (3) + Tract (6) |
| Block Group | 12 digits | State (2) + County (3) + Tract (6) + BG (1) |
| Census Block | 15 digits | State (2) + County (3) + Tract (6) + Block (4) |
| Place | 7 digits | State (2) + Place (5) |
| ZCTA (ZIP area) | 5 digits | ZCTA code (5) — not a ZIP code |
The Most Common Gotcha: Tract GEOIDs in Census Data
When you download Census ACS data at the tract level, the GEO_ID column typically looks like
1400000US53033005302 — with a prefix indicating the geographic summary level.
The numeric part after US is the GEOID. Strip the prefix before joining to other datasets.
import pandas as pd
df = pd.read_csv('census_tract_data.csv', dtype=str)
# GEO_ID looks like "1400000US53033005302" — strip the prefix
df['geoid'] = df['GEO_ID'].str.replace(r'^.*US', '', regex=True)
# Now extract components
df['state_fips'] = df['geoid'].str[:2]
df['county_fips'] = df['geoid'].str[:5]
df['tract_code'] = df['geoid'].str[5:11]ZCTAs Are Not ZIP Codes
One additional source of confusion: the Census Bureau uses ZCTAs (ZIP Code Tabulation Areas), which are Census-defined approximations of USPS ZIP codes. ZCTA codes look like ZIP codes (5 digits) but are not the same — a few thousand ZIP codes have no ZCTA equivalent, and ZCTA boundaries can differ from the actual postal delivery area.
For geographic analysis, always use FIPS codes. ZCTAs are an approximation of ZIP codes, not a substitute for county FIPS codes.