Most Americans recognize the standard 5-digit ZIP code, but you've probably seen mail labeled with something like 10001-2452 or 90210-3870. That extra hyphen and four digits are part of a system the U.S. Postal Service introduced more than four decades ago — and despite being mostly invisible to consumers, ZIP+4 is what makes modern bulk mail discounts and same-day fulfillment possible.
This guide explains what each of those four digits represents, how USPS assigns them, who's required to use them, and how you can look up the ZIP+4 for any U.S. address.
ZIP+4 (officially called the "ZIP+4 Code" by USPS) is a 9-digit version of the ZIP code that adds four extra digits after a hyphen. The format is always:
NNNNN-NNNN — for example, 20500-0001 (The White House)
The first five digits are the standard ZIP code that identifies a regional sectional center facility (SCF) and a specific delivery post office. The four added digits — sometimes called the "add-on" or "plus-four" — narrow delivery down to a much smaller area, often a single block, a side of a street, a high-rise floor, or even a specific PO Box.
USPS rolled out ZIP+4 on October 1, 1983, twenty years after the original ZIP code system launched in 1963. The motivation was simple: by the early 1980s, mail volume had again exploded, especially direct marketing and bulk business mail. Adding four digits gave USPS sorting machines enough granularity to route a letter directly to the carrier's individual route, eliminating an entire layer of manual sorting.
Initially, the system was unpopular. A 1983 New York Times piece reported that some businesses called it "an unnecessary burden," and USPS spent millions promoting it. Adoption only took off when USPS began offering meaningful postage discounts — discounts that still exist today and remain the primary reason bulk mailers use ZIP+4 religiously.
The breakdown of a full 9-digit code looks like this:
| Digits | Meaning |
|---|---|
| 1st | National region (0–9) |
| 2nd–3rd | Sectional Center Facility (SCF) |
| 4th–5th | Local delivery area or post office |
| 6th–7th | Delivery sector (a few blocks, a building, or a group of PO Boxes) |
| 8th–9th | Delivery segment (one side of a street, a floor, a department, or a single firm) |
So if your address is 1600 Pennsylvania Ave NW, Washington DC 20500-0003, the "00" tells USPS the delivery sector and "03" identifies the specific delivery segment within The White House complex.
Plus-four codes are not random. USPS maintains a database — called the City State Product — that maps every address in the country to a specific ZIP+4. The codes are assigned based on:
The codes change occasionally. New construction, route reorganizations, and address renumbering all trigger updates. USPS publishes revisions to the City State Product every two months.
For everyday personal mail, ZIP+4 is completely optional. A letter addressed with just the 5-digit ZIP code will arrive normally. However, ZIP+4 is required or strongly incentivized in several situations:
To qualify for the lowest Marketing Mail and First-Class Presort postage rates — which can save large mailers as much as 30 to 40 percent — USPS requires that 95 percent or more of the mailing be addressed with valid ZIP+4 codes. This is why every catalog or piece of junk mail you receive includes the full 9-digit code.
Companies that send mail must process their address lists through Coding Accuracy Support System (CASS) certified software, which appends and validates ZIP+4 codes. Without CASS certification, mailers don't qualify for automation discounts.
Some federal mailings — including IRS correspondence, Social Security checks, and selective service notices — use ZIP+4 to ensure delivery to specific government office buildings rather than general post offices.
The official source for any address's full ZIP+4 is the USPS ZIP Code Lookup tool at usps.com. Enter a complete street address and USPS returns the standardized address along with the full 9-digit code. Third-party tools, including Zip Instant, can help you find the 5-digit ZIP for any city, but for the precise plus-four, USPS itself is the authoritative source — because USPS owns and updates the underlying City State Product.
Sort of, but not as precisely as people imagine. A typical residential ZIP+4 covers around 10 to 20 households on the same side of a street. It's much more granular than a 5-digit ZIP (which can cover thousands of homes), but it doesn't pinpoint a single house or apartment unit.
It is not. Federal law does not require senders or recipients to use ZIP+4. The only "requirement" is contractual — businesses agreeing to USPS's automation rate program must use it.
It can. In large apartment complexes, different floors or wings may have separate ZIP+4 codes. Office buildings often have separate codes for individual major tenants.
For individual letters, the speed difference is negligible. USPS sorting machines read the full 9-digit code when present and skip a manual carrier-sortation step, but for personal mail this saves only minutes of processing time, not days.
For business mail, the difference compounds: a presorted, ZIP+4-coded mailing of 100,000 pieces can move through USPS facilities a full day faster than the same mailing without coding, because it bypasses several handling steps. Combined with the postage discount, this is why every direct mail program uses ZIP+4.
USPS has explored several proposals to extend or replace ZIP+4, including 11-digit ZIP codes (which already exist internally for delivery point coding) and full alphanumeric formats like the United Kingdom's. None have moved beyond planning stages. The 9-digit ZIP+4 remains deeply integrated into mailing software, government systems, and address databases nationwide — and like the original ZIP code, it's likely to stay around for decades.