US ZIP Validator

US ZIP code: Zone Improvement Plan, five digits plus four

ZIP stands for Zone Improvement Plan, introduced by the US Postal Service on 1 July 1963. The basic code is 5 digits; the extended ZIP+4 form (NNNNN-NNNN), added in 1983, pins delivery down to a city block, a single building or a PO box group. The validator on this page accepts both 90210 and 90210-1234, checking the structural format — five digits, optional hyphen, four digits.

There is no check digit in the printed code (the old POSTNET barcode carried one, but the digits themselves don't). The five digits cascade from a broad region to a specific post office, and the +4 adds the fine-grained delivery segment.

Reading the five digits

• Digit 1: group of states (e.g. 0 New England/NJ, 1 NY/PA, 9 CA/Pacific).
• Digits 1–3: the Sectional Center Facility (SCF) — a regional sorting hub.
• Digits 4–5: the specific post office or delivery area.
• +4: a block face, a floor of a building, a cluster of PO boxes, or a single high-volume mailer.

Where it shows up

• Address forms: ZIP lookup auto-fills city + state; the standard US checkout flow.
• Tax & shipping: sales-tax rate, shipping zone and carrier rating all key off the ZIP.
• Geo & demographics: the Census Bureau publishes data by ZCTA (ZIP Code Tabulation Area), an approximation of ZIP coverage.
• Mock data / testing: generating format-valid codes for fixtures and form QA.

Gotchas

• Leading zeros: New England ZIPs start with 0 (e.g. 02101 Boston) — store as a string, never an integer.
• ZIP ≠ city/county boundary: ZIPs are delivery routes, not geographic areas; one ZIP can cross county or even state lines, and a ZCTA only approximates it.
• Unique ZIPs: some ZIPs belong to a single entity (e.g. 20500 the White House, 12345 General Electric in Schenectady).
• Format-valid ≠ real: 00000 passes the regex but isn't assigned; the lowest real ZIP is 00501 (an IRS facility in Holtsville, NY).

FAQ

What does the +4 add? It narrows delivery from a post office to a specific segment — one side of a street, a single building, or a PO box block — speeding up automated sorting and qualifying mailers for postage discounts.

Is the hyphen required? Only to separate the 5 and the +4; 90210 1234 or 902101234 normalise to the same value. The base 5-digit code is always sufficient for delivery.

Is there a check digit? Not in the human-readable code. Validity is by lookup against USPS data; the legacy POSTNET barcode added a correction digit separately.