The short answer
A ZIP Code is the five-digit code the US Postal Service uses to sort mail by delivery area. ZIP stands for Zone Improvement Plan, and USPS introduced it on July 1, 1963. The first digit marks a broad region of the country, the first three digits the regional processing center, and the last two the local Post Office or delivery area. ZIP+4 adds a hyphen and four digits that narrow delivery to a block, a building, or a group of PO Boxes.
What is a ZIP Code?
A ZIP Code is the five-digit number the US Postal Service assigns to delivery areas so machines and postal workers can sort mail quickly. It sits at the end of the last line of a US address, after the city and the two-letter state abbreviation, as in “SPRINGFIELD IL 62701.”
The Domestic Mail Manual, the rulebook USPS publishes for mailers, describes the ZIP Code system as “a numbered coding system that facilitates efficient mail processing.” USPS assigns every code. Every Post Office gets at least one five-digit ZIP Code, and larger Post Offices may get two or more, with a separate code for each delivery unit.
A ZIP Code is a mail-routing code. It tells USPS where to send a letter, and one ZIP Code can cover more than one city name. The USPS ZIP Code Lookup has a “Cities by ZIP Code” search that lists every city a code covers and notes that using the recommended city name helps mail move faster.
What does ZIP stand for, and when did ZIP Codes start?
ZIP stands for Zone Improvement Plan. That’s the expansion in the Domestic Mail Manual and in the USPS history timeline “The Time of Our Life,” which says the Zone Improvement Plan’s ZIP Code was introduced July 1, 1963, “to speed up mail processing.” Some USPS Postal Facts pages say “Zoning Improvement Plan” instead, so you’ll see both versions in official sources.
USPS Postal Facts says the code was launched “to better process and deliver increasing volumes of U.S. Mail.” The same USPS timeline says that in 1965 the Post Office Department became the first postal system to use optical scanners to read ZIP Codes, sorting up to 36,000 letters per hour. The four-digit add-on, ZIP+4, came in 1983.
USPS Postal Facts now counts 41,554 ZIP Codes. They run from 00501, which belongs to the Internal Revenue Service in Holtsville, New York, to 99950 in Ketchikan, Alaska.
What do the five digits of a ZIP Code mean?
Read a ZIP Code from left to right and it narrows down from a big region to a single delivery area. USPS Postal Facts explains that the first digit stands for a general geographic area of the country, from “0” in the east to “9” in the west. The USPS ZIP Code FAQ says the first three digits together identify the Sectional Center Facility, the processing plant that serves a region, or a large city. The last two digits point to the Post Office or delivery area within it.
So a ZIP Code starting with 0 is in the east, and Ketchikan, Alaska’s 99950 starts with 9. The table uses a made-up code, 12345-6789, the same example the USPS FAQ uses (USPS says the real 12345 is a unique ZIP Code for General Electric in Schenectady, New York).
| Digits | Example | What they identify |
|---|---|---|
| First digit | 1 | A general geographic area of the country, numbered 0 in the east to 9 in the west |
| First three digits | 123 | The Sectional Center Facility (regional processing plant) or a large city |
| Fourth and fifth digits | 45 | The Post Office facility or delivery area |
| Sixth and seventh digits (first two after the hyphen) | 67 | A sector, such as several blocks |
| Eighth and ninth digits | 89 | A segment, such as one side of a street, a building, or a group of PO Boxes |
What is ZIP+4?
ZIP+4 is the full nine-digit ZIP Code: the five-digit code, a hyphen, and four more digits, such as 12345-6789. The Domestic Mail Manual calls it “the most complete ZIP Code.” The first two of the extra digits identify a sector and the last two a smaller segment. Together they can narrow delivery down to one side of a street between intersections, a single building, a floor or a firm within a building, a span of boxes on a rural route, or a group of PO Boxes served by one USPS employee.
The hyphen is required. The USPS ZIP Code FAQ says a ZIP+4 code must be five digits, a hyphen, and four digits, and that improperly labeled mail “may progress slower through the mail system.” Don’t write 123456789 or 12345 6789.
ZIP+4 matters most for businesses that print barcodes and presort mail to qualify for automation prices, which require address lists coded with USPS-certified address-matching software. For a personal letter, the key is a complete, correct address with the right ZIP Code; our mailing address guide shows the full format.
How do I find my ZIP+4?
The free USPS ZIP Code Lookup at tools.usps.com finds ZIP Codes three ways:
- By address: enter a home or business street address, city, and state to see the specific ZIP Code for that address. A USPS fact sheet explains that searching by address returns the full ZIP+4.
- By city and state: enter a city and state to see all the ZIP Codes for that city.
- Cities by ZIP Code: enter a ZIP Code to see the cities it covers, including the city name USPS recommends.
Why does the ZIP Code Lookup change how my address is written?
The tool shows your address in USPS standardized format, so the result may look different from what you typed. The USPS FAQ recommends using the standardized version on mail so sorting machines can process it without delays.
If the tool doesn’t recognize your address, USPS says it may have been typed wrong, may not be a valid address, or may be new construction. Check the address, try again, and contact your local Post Office if a valid address still doesn’t show up. USPS also says ZIP Code changes can take 30 to 60 days to appear in the lookup tool.
What is the delivery point barcode?
The delivery point barcode goes one step beyond ZIP+4 and identifies a single delivery point, such as one house or one mailbox. The Domestic Mail Manual says its numeric form is five digits, a hyphen, and seven digits: the ZIP+4 code with three digits added directly after it.
Today that code is printed as part of the Intelligent Mail barcode, the row of tall and short bars on business mail. The Domestic Mail Manual says the barcode encodes 31 digits: a 20-digit tracking code and an 11-digit routing code. The routing code can hold a five-digit ZIP Code, a ZIP+4 code, or a full delivery point code, and letters mailed at automation prices must carry a delivery point code that matches the address.
Individual senders never need to create one. Publication 28, USPS’s addressing standard, only requires that a preprinted delivery point barcode be correct for the delivery address, city, state, and ZIP+4 Code on the mailpiece.
What are unique ZIP Codes and PO Box ZIP Codes?
Some ZIP Codes don’t cover a neighborhood at all. The USPS ZIP Code FAQ says a unique ZIP Code is “assigned to a company, government agency, or entity with sufficient mail volume,” based on how much letter mail it receives daily, whether numbers are available in the area, and a USPS cost-benefit analysis. The IRS’s 00501 in Holtsville, New York, and General Electric’s 12345 in Schenectady are two USPS examples.
PO Boxes get ZIP Codes too. Under the Domestic Mail Manual, a ZIP+4 code can identify a group of PO Boxes that one USPS employee serves. USPS says ZIP Codes for PO Boxes in the same city usually don’t change and suggests asking your local Post Office how they’re assigned. Use the ZIP Code that goes with the box, which the ZIP Code Lookup returns when you search the PO Box address. Our PO Box address guide shows the format.
What’s the difference between a ZIP Code and a postal code?
A postal code is the general term for any country’s mail-sorting code. ZIP Code is the US version, and the name is a USPS trademark. The USPS FAQ describes foreign postal codes as “similar to the ZIP Code used in the USA,” and says USPS doesn’t keep information on them. To find a postal code in another country, check that country’s postal service.
The biggest visible difference is format. Canada and the United Kingdom use letters and numbers, while US ZIP Codes are numbers only.
| Country | Name | Format | Example | Parts |
|---|---|---|---|---|
| United States | ZIP Code (USPS) | 5 digits, or 5 digits, a hyphen, and 4 digits | 12345-6789 | Sectional center and delivery area, plus sector and segment |
| Canada | Postal code (Canada Post) | Six characters in the form ANA NAN (A = letter, N = number), with one space | ANA NAN | Forward Sortation Area (first three) and Local Delivery Unit (last three) |
| United Kingdom | Postcode (Royal Mail) | Outward code of 2–4 characters, a space, inward code of 3 characters | PO15 5RR | Outward code and inward code |
How do you write a ZIP Code on an envelope?
The ZIP Code goes at the end of the last line of the address, after the city and state. Publication 28 calls for at least one space between the city, the two-letter state abbreviation, and the ZIP+4 Code, and prefers two spaces between the state and the ZIP Code.
Include the hyphen in a ZIP+4. Write the ZIP Code for the address element directly above the city line: if an address shows both a street address and a PO Box, the Domestic Mail Manual says mail goes to the one right above the city and state, and the ZIP Code has to match that line. Canada Post asks for its postal codes in uppercase with one space and no hyphens. Our envelope guide covers return addresses, placement, and stamps.
Sources & further reading
Product details checked against the IRIS API contract when this article was published. Platform references can change; consult the linked documentation when implementing.
- USPS Domestic Mail Manual 602, Addressing
- USPS Domestic Mail Manual 204, Barcode Standards
- USPS FAQ: ZIP Code – The Basics
- USPS Postal Facts: Number of ZIP Codes
- USPS Postal Facts: 1963 – ZIP Code introduced
- USPS history: The Time of Our Life, 1775–2025 (PDF)
- USPS ZIP Code Lookup
- USPS Postal Bulletin 22227: Fact Sheet, Look Up a ZIP Code
- USPS Publication 28, 224: Format
- USPS Publication 28, 226: Preprinted Delivery Point Barcodes
- Canada Post: Addressing guidelines, postal codes
- Canada Post: Addressing guidelines, important information
- Office for National Statistics: Postal geographies
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