ZIP Code 91001 — Altadena, CA
91001 is a standard (street delivery) ZIP code serving Altadena, California, in Los Angeles 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 91001 at a glance
| ZIP code | 91001 |
|---|---|
| Primary city | Altadena |
| State | California (CA) |
| County | Los Angeles FIPS 06037 |
| ZIP type | Standard (street delivery) |
| Time zone | America/Los_Angeles |
| Population | 35,475 |
| Coordinates | 34.195220, -118.138660 |
How ZIP+4 works in 91001
The four digits after the hyphen encode a delivery segment inside 91001. 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 91001 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 91001.
| ZIP | City | Distance |
|---|---|---|
| 91104 | Pasadena, CA | 2.1 mi |
| 91103 | Pasadena, CA | 2.4 mi |
| 91101 | Pasadena, CA | 3.3 mi |
| 91107 | Pasadena, CA | 3.9 mi |
| 91106 | Pasadena, CA | 4 mi |
| 91011 | La Canada Flintridge, CA | 4 mi |
| 91125 | Pasadena, CA | 4 mi |
| 91105 | Pasadena, CA | 4.1 mi |
| 91206 | Glendale, CA | 4.9 mi |
| 91108 | San Marino, CA | 5.2 mi |
| 91020 | Montrose, CA | 5.4 mi |
| 91024 | Sierra Madre, CA | 5.5 mi |
Questions about 91001
What is the ZIP+4 code for 91001?
91001 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 91001, you need to look up a specific street address to get them.
What city is 91001?
91001 is assigned to Altadena, California, in Los Angeles 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.