ZIP Code 78664 — Round Rock, TX
78664 is a standard (street delivery) ZIP code serving Round Rock, Texas, in Williamson 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 78664 at a glance
| ZIP code | 78664 |
|---|---|
| Primary city | Round Rock |
| State | Texas (TX) |
| County | Williamson FIPS 48491 |
| ZIP type | Standard (street delivery) |
| Time zone | America/Chicago |
| Population | 62,060 |
| Coordinates | 30.504610, -97.645830 |
How ZIP+4 works in 78664
The four digits after the hyphen encode a delivery segment inside 78664. 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 78664 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.
Other ZIP codes in Round Rock
Nearby ZIP codes
Showing the closest twelve. See every ZIP code within 25 miles of 78664.
| ZIP | City | Distance |
|---|---|---|
| 78665 | Round Rock, TX | 3.2 mi |
| 78728 | Austin, TX | 4.3 mi |
| 78681 | Round Rock, TX | 4.9 mi |
| 78660 | Pflugerville, TX | 5.2 mi |
| 78717 | Austin, TX | 6.5 mi |
| 78727 | Austin, TX | 6.7 mi |
| 78634 | Hutto, TX | 7.2 mi |
| 78729 | Austin, TX | 7.3 mi |
| 78753 | Austin, TX | 8.5 mi |
| 78758 | Austin, TX | 8.8 mi |
| 78759 | Austin, TX | 9.8 mi |
| 78754 | Austin, TX | 10.2 mi |
Questions about 78664
What is the ZIP+4 code for 78664?
78664 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 78664, you need to look up a specific street address to get them.
What city is 78664?
78664 is assigned to Round Rock, Texas, in Williamson 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.