Japan Postal Code Validator

Japan uses a precise, nationally standardised 7-digit postal code system run by Japan Post (日本郵便). Every address in the country — from downtown Tokyo skyscrapers to remote Okinawan islands — is covered by a code written in the form NNN-NNNN: three digits, a hyphen, then four digits, often prefixed by the post-office symbol 〒. This tool validates any such code entirely in your browser, with no data ever leaving your device.

How the Japan postal code system works

Japan Post introduced the modern 7-digit scheme in February 1998, expanding an earlier 3-digit and then 5-digit framework to provide enough granularity for precise machine sorting. Each code breaks into two parts:

• District block (first 3 digits): A coarse geographic region, almost always a single prefecture or a named section of a large metropolitan area. Codes 100-199 cover Tokyo; 530-599 cover Osaka City; 900-909 cover Okinawa Prefecture.
• Delivery area (last 4 digits): A specific neighbourhood, building cluster, or PO box within that district. Populated areas may have many hundreds of delivery-area codes within a single 3-digit block.

Validation here applies three rules in order. First, separators (hyphens, spaces, fullwidth dashes) are stripped and the remaining characters must all be ASCII digits. Second, the result must be exactly 7 digits — too few or too many means the code is wrong or was copied incompletely. Third, the first digit must be 1 through 9: no valid Japan Post assignment begins at 0, so a leading zero indicates a foreign postcode, a truncated value, or a data-entry error. Finally the 3-digit prefix is matched against a lookup table of all currently assigned prefecture blocks; an unrecognised prefix is flagged as unknown (either unassigned or issued after this tool’s last update).

Because the scheme carries no checksum digit, those are the only structural tests available without querying Japan Post’s live database. This is by design: the tool stays completely offline and private.

Worked example

Suppose you need to validate the code for the Tokyo Imperial Palace area: 100-0001.

1. Strip the hyphen to get the 7-digit string 1000001.
2. Length check: 7 digits — pass.
3. First digit is 1 (non-zero) — pass.
4. 3-digit prefix 100 maps to Tokyo in the prefecture table — pass.
5. Formatted output: 100-0001, prefecture: Tokyo.

Now try 000-0000 (all zeros). After stripping: 0000000. First digit is 0 — the validator immediately rejects it with “No valid Japan postal code starts with 0.”

Or try 999-9999. Digits: 7, first digit: 9 (non-zero), but prefix 999 is not assigned in the known block table, so the result is “Unrecognised prefix” — the code is plausible in structure but cannot be confirmed as a real locality.

Every check runs locally — nothing is uploaded or stored.