Congressional Districts and ZIP Codes

## Two Different Maps

Congressional districts and ZIP codes are drawn for entirely different purposes. Congressional districts represent roughly equal populations (~760,000 per district) for legislative representation. ZIP codes represent mail delivery efficiency.

The result: the two boundary systems rarely align.

## How They Overlap

| Scenario | Frequency |
|----------|----------|
| ZIP entirely in one district | ~60% of ZIPs |
| ZIP split across 2 districts | ~30% of ZIPs |
| ZIP split across 3+ districts | ~10% of ZIPs |

In dense urban areas, a single ZIP code may span 4-5 congressional districts. In rural areas, a single district may contain hundreds of ZIP codes.

## Why This Matters

When constituents contact Congress or political campaigns target voters by ZIP code, the mismatch causes problems:

- A constituent in a split ZIP may not know which district they belong to
- Campaign mailers sent to a ZIP may reach voters in the wrong district
- "Find your representative" tools must use full addresses, not just ZIPs

## The Correct Approach

To determine the correct congressional district:

1. **Full address** → geocode to latitude/longitude
2. **Point-in-polygon** → overlay with district shapefiles
3. **Confirm** → validate against the Census block assignment file

Using ZIP code alone gives the wrong result up to 40% of the time in urban areas.

## Redistricting Cycles

Congressional districts are redrawn every 10 years after the decennial Census. The most recent redistricting cycle (2021-2022) changed boundaries in 46 states, affecting the district assignment of thousands of ZIP codes.

## At-Large Districts

Seven states have a single at-large congressional district: Alaska, Delaware, Montana (gained a second seat in 2022, so now has two), North Dakota, South Dakota, Vermont, and Wyoming. In these states, every ZIP code maps to the same district — making the ZIP-to-district problem trivial.