Counties, FIPS Codes, and ZIP Codes

## Three Overlapping Systems

Counties, FIPS codes, and ZIP codes are three geographic systems that overlap imperfectly. Understanding their relationship is essential for data analysis and geographic matching.

## What Are FIPS Codes?

FIPS (Federal Information Processing Standards) codes are numeric identifiers assigned to geographic entities by the federal government:

| Level | Format | Example |
|-------|--------|--------|
| State | 2 digits | 36 = New York |
| County | 5 digits (state + county) | 36061 = New York County (Manhattan) |
| Census Tract | 11 digits | 36061000100 |
| Census Block | 15 digits | 360610001001000 |

## ZIP-County Mismatch

ZIP codes were designed for mail routing; counties were drawn for governance. The result:

- **~35% of ZIP codes** span two or more counties
- In extreme cases, a ZIP can span 3-4 counties
- County-level data (elections, courts, taxes) cannot be cleanly mapped to ZIPs

## Crosswalk Files

The Census Bureau and HUD publish ZIP-to-county crosswalk files that list all county overlaps for each ZIP:

| ZIP | County FIPS | County Name | Residential Ratio |
|-----|------------|-------------|-------------------|
| 27510 | 37019 | Brunswick, NC | 0.65 |
| 27510 | 37047 | Columbus, NC | 0.35 |

The residential ratio indicates what fraction of the ZIP's population lives in each county.

## Practical Applications

ZIP-county mapping is needed for:

- **Tax jurisdiction** — Sales tax rates vary by county
- **Election administration** — Voter registration requires county, not ZIP
- **Healthcare** — Hospital service areas align with counties
- **Insurance** — Auto insurance rates often set at the county level
- **Government reporting** — Federal programs track outcomes by county FIPS

## Best Practice

When you need to assign a ZIP code to a single county (many databases require 1:1 mapping), use the county with the highest residential ratio. But document this as an approximation — the true assignment requires address-level geocoding.