ZIP Code 78666 — San Marcos, TX
78666 is a standard (street delivery) ZIP code serving San Marcos, Texas, in Hays 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 78666 at a glance
| ZIP code | 78666 |
|---|---|
| Primary city | San Marcos |
| State | Texas (TX) |
| County | Hays FIPS 48209 |
| ZIP type | Standard (street delivery) |
| Time zone | America/Chicago |
| Population | 88,583 |
| Coordinates | 29.882600, -97.979740 |
How ZIP+4 works in 78666
The four digits after the hyphen encode a delivery segment inside 78666. 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 78666 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 78666.
| ZIP | City | Distance |
|---|---|---|
| 78655 | Martindale, TX | 9.6 mi |
| 78656 | Maxwell, TX | 10.3 mi |
| 78670 | Staples, TX | 11.5 mi |
| 78640 | Kyle, TX | 12.2 mi |
| 78622 | Fentress, TX | 14.1 mi |
| 78130 | New Braunfels, TX | 14.2 mi |
| 78676 | Wimberley, TX | 15 mi |
| 78132 | New Braunfels, TX | 15 mi |
| 78623 | Fischer, TX | 15.3 mi |
| 78610 | Buda, TX | 15.9 mi |
| 78133 | Canyon Lake, TX | 15.9 mi |
| 78619 | Driftwood, TX | 16.1 mi |
Questions about 78666
What is the ZIP+4 code for 78666?
78666 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 78666, you need to look up a specific street address to get them.
What city is 78666?
78666 is assigned to San Marcos, Texas, in Hays 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.