ZIP Code 21791 — Union Bridge, MD
21791 is a standard (street delivery) ZIP code serving Union Bridge, Maryland, in Frederick County. To mail something here you only need these five digits — but the four-digit extension narrows delivery to a single point, and it is what USPS bulk postage discounts are priced on.
ZIP code 21791 at a glance
| ZIP code | 21791 |
|---|---|
| Primary city | Union Bridge |
| State | Maryland (MD) |
| County | Frederick FIPS 24021 |
| ZIP type | Standard (street delivery) |
| Time zone | America/New_York |
| Population | 5,340 |
| Coordinates | 39.542670, -77.187130 |
How ZIP+4 works in 21791
The four digits after the hyphen encode a delivery segment inside 21791. In practice one segment usually means:
- One side of one block — odd numbers on a street between two cross streets
- A single large building, or one floor of it
- A group of PO boxes, sometimes a single box
- A single business that receives enough mail to warrant its own code
That is why 21791 has no single ZIP+4: two neighbors across the street from each other usually have different ones. Enter a street address above and we will return the exact nine-digit code along with the standardized USPS address format.
Nearby ZIP codes
Showing the closest twelve. See every ZIP code within 25 miles of 21791.
| ZIP | City | Distance |
|---|---|---|
| 21776 | New Windsor, MD | 4.7 mi |
| 21762 | Libertytown, MD | 5.4 mi |
| 21757 | Keymar, MD | 5.5 mi |
| 21798 | Woodsboro, MD | 6 mi |
| 21787 | Taneytown, MD | 8.9 mi |
| 21793 | Walkersville, MD | 9.1 mi |
| 21778 | Rocky Ridge, MD | 9.5 mi |
| 21701 | Frederick, MD | 10.3 mi |
| 21771 | Mount Airy, MD | 10.3 mi |
| 21774 | New Market, MD | 10.4 mi |
| 21157 | Westminster, MD | 10.9 mi |
| 21158 | Westminster, MD | 10.9 mi |
Questions about 21791
What is the ZIP+4 code for 21791?
21791 is the five-digit ZIP code. The ZIP+4 adds four more digits that identify a specific delivery point — usually one side of a city block, a single building, or a group of PO boxes. Because those four digits change from address to address within 21791, you need to look up a specific street address to get them.
What city is 21791?
21791 is assigned to Union Bridge, Maryland, in Frederick County.
Do I need to use the ZIP+4?
It is optional for ordinary mail, but it speeds up sorting and it is required for most USPS bulk-mail postage discounts. Many address-verification and e-commerce systems also use ZIP+4 to confirm a delivery point exists.
Need thousands of these?
Upload a CSV and get ZIP+4, county, carrier route, and delivery-point data appended to every row — or call the API directly.