Format and history
The standard 5-digit ZIP code identifies a post office or delivery area. The 4-digit suffix in ZIP+4, separated by a hyphen, identifies a smaller routing unit within that ZIP.
USPS introduced ZIP+4 on October 1, 1983 after a multi-year pilot and a 1981 announcement. The agency had previously experimented with similar ideas in the 1970s as mail volume continued to climb.
Although the original USPS plan was to make ZIP+4 mandatory by the end of the 1980s, public resistance from consumers and small mailers led the agency to keep it optional for individual senders. Large mailers, however, must use ZIP+4 to qualify for automation rates.
What the 4-digit suffix actually points to
The four digits resolve to one of several routing units, depending on the address type. For a residential block, the suffix usually identifies a block face, meaning one side of one street between two intersections.
For a large building, such as a high-rise office tower, the suffix can identify a specific floor or even a specific mail stop. For PO Boxes, the suffix identifies a contiguous group of boxes within the post office.
Some unique ZIP codes, assigned to a single large business, use the ZIP+4 suffix to route mail to individual departments or buildings within the recipient organization.
Why USPS wants you to use ZIP+4
Automated mail sorting equipment reads addresses through Optical Character Recognition and applies barcodes for routing. Mail that includes a valid ZIP+4 can be sorted to the carrier-route level by machine, skipping manual handling.
USPS gives volume discounts to mailers who pre-sort with ZIP+4. For a business sending 50,000 invoices a month, the discount is material: single-digit cents per piece, multiplied by tens of thousands of pieces.
From the recipient's point of view, mail addressed with ZIP+4 is statistically less likely to be misrouted or returned as undeliverable.
How to find your ZIP+4
USPS hosts a free ZIP+4 lookup tool at the address USPS.com under "Find a ZIP Code." Enter a street address and the tool returns the full 9-digit code.
Most address verification APIs (including those from USPS Web Tools, Smarty, Lob, and Melissa) return ZIP+4 as part of their standardized output. Customer relationship management systems and ecommerce checkout tools often append ZIP+4 silently during address standardization.
Privacy and data implications
A ZIP+4 is far more precise than a ZIP code. In a residential context, a single suffix may correspond to as few as 10 to 30 households on one side of one block.
Researchers and privacy regulators treat ZIP+4 with more care than a 5-digit ZIP because it can be combined with other attributes (age, gender, occupation) to re-identify individuals in supposedly anonymized data sets. The U.S. Department of Health and Human Services HIPAA Safe Harbor de-identification standard, for example, requires only the first three digits of a ZIP and never the full ZIP+4.