United States
United States ZIP Code Format Explained
Learn the structure of a United States ZIP code and ZIP+4, what each part means, valid format examples and the mistakes that cause problems.
Updated 14 October 2026 · 3 min read
A United States ZIP code is a five-digit number, optionally followed by a hyphen and four more digits. The five-digit form identifies a delivery area; the four-digit extension, called ZIP+4, points to a smaller segment such as one side of a street, a group of mailboxes or a single organization.
The two forms
| Form | Pattern | Generic example |
|---|---|---|
| Standard ZIP | 5 digits | 12345 |
| ZIP+4 | 5 digits, hyphen, 4 digits | 12345-6789 |
Those example values are generic and only show the shape. Always take the real value from the address of the place you need.
What the digits mean
The five-digit code is built in layers. The first digit identifies a broad group of states, running from the north-east (low numbers) to the west (high numbers). The next two digits narrow it to a regional sorting facility, and the last two digits point to a specific post office or delivery area within that region.
The four digits after the hyphen are an add-on. In general terms, the first two identify a delivery sector and the last two a delivery segment within it. You do not need to understand this to use a ZIP code, but it explains why two neighbouring buildings can share five digits and differ in the extension.
ZIP codes are not boundaries
ZIP codes are designed for mail delivery routes. They are not official geographic areas, and a single ZIP code can in rare cases cross a state line. Some ZIP codes belong to a single university, government office or large company. For this reason, a ZIP code alone is a poor way to describe a precise place; an exact point is better described by a location code such as UPCS.
Common mistakes
- Dropping a leading zero. Many north-eastern ZIP codes begin with zero. Spreadsheets often treat the value as a number and remove it, turning a five-digit code into four. Store ZIP codes as text.
- Using a space or letters. A ZIP code contains only digits, plus the hyphen in ZIP+4.
- Mixing up the hyphen position. The hyphen goes after the fifth digit, not before.
- Guessing the extension. If you are not certain of the four digits, leave them out. A wrong extension is worse than none.
- Assuming one ZIP per city. Large cities have many ZIP codes, and small towns may share one.
Related pages
If you are not sure which code applies to an address, read how to find a ZIP code. For the whole address layout, see United States address format. An exact-location code is described in the country overview.
Frequently asked questions
How many digits does a US ZIP code have?
A standard ZIP code has five digits. The extended ZIP+4 form has nine digits, written as five digits, a hyphen and four more digits.
Is the ZIP+4 extension required?
Usually not. Mail normally arrives with only the five-digit ZIP code, and most websites accept either form. The extra four digits mainly help with automated sorting.