ZIP Code 51003 — Alton, IA
51003 is a standard (street delivery) ZIP code serving Alton, Iowa, in Sioux 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 51003 at a glance
| ZIP code | 51003 |
|---|---|
| Primary city | Alton |
| State | Iowa (IA) |
| County | Sioux FIPS 19167 |
| ZIP type | Standard (street delivery) |
| Time zone | America/Chicago |
| Population | 1,938 |
| Coordinates | 42.983550, -95.986420 |
How ZIP+4 works in 51003
The four digits after the hyphen encode a delivery segment inside 51003. 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 51003 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 51003.
| ZIP | City | Distance |
|---|---|---|
| 51041 | Orange City, IA | 5.4 mi |
| 51022 | Granville, IA | 7 mi |
| 51238 | Hospers, IA | 8 mi |
| 51036 | Maurice, IA | 10.3 mi |
| 51045 | Oyens, IA | 11.8 mi |
| 51050 | Remsen, IA | 13.1 mi |
| 51250 | Sioux Center, IA | 13.4 mi |
| 51234 | Boyden, IA | 14.6 mi |
| 51231 | Archer, IA | 15 mi |
| 51201 | Sheldon, IA | 15.1 mi |
| 51046 | Paullina, IA | 15.5 mi |
| 51031 | Le Mars, IA | 15.9 mi |
Questions about 51003
What is the ZIP+4 code for 51003?
51003 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 51003, you need to look up a specific street address to get them.
What city is 51003?
51003 is assigned to Alton, Iowa, in Sioux 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.