ID Card Age Calculator

The ID Age Calculator extracts the birth date encoded in a Chinese resident identity card number and calculates the corresponding age. It helps check form entries, membership records, eligibility thresholds, school or exam data, and demographic tables without manually reading the date segment from the ID. The tool only works from the information already present in the number; it is not an official identity verification service and should not encourage storing full ID numbers unnecessarily. For privacy-sensitive or legally binding checks, the original document and the organization’s verification process remain authoritative.

The 18-digit Chinese resident identity card defined in GB 11643-1999 packs five fields into a fixed-width string: digits 1-6 are the administrative division code from GB/T 2260, digits 7-14 are the birth date in YYYYMMDD form, digits 15-17 are the issuing sequence number, and digit 18 is an ISO 7064 MOD 11-2 check character. The 15-digit pre-1999 format omits the century prefix and the check digit, so upgrading it means inserting '19' before the YY block and then computing the trailing checksum. The checksum is the heart of validation. Each of the first 17 digits is multiplied by its weight from the sequence 7-9-10-5-8-4-2-1-6-3-7-9-10-5-8-4-2, the weighted sum is taken modulo 11, and the remainder is mapped through the table '1-0-X-9-8-7-6-5-4-3-2'. A remainder of 2 yields the literal character 'X' rather than a digit, which is why the last position must be parsed case-insensitively. Gender is decoded from the parity of digit 17 alone (odd is male, even is female), and the age is the difference of calendar years minus one if today is earlier in the year than the encoded birthday. What the algorithm cannot prove is whether a real person owns the number. A single transposition that keeps the checksum valid will still pass, retired region codes from administrative mergers may no longer resolve in GB/T 2260, and the parity-bit gender reflects the issuing window, not current self-identification. Treat the parse as cheap structural validation and defer real identity proofing to the issuing authority.

• Checksum: ISO 7064:1983 MOD 11-2 over the first 17 digits, with weights 7-9-10-5-8-4-2-1-6-3-7-9-10-5-8-4-2 and the remainder table '10X98765432'.
• Date field: digits 7-14 use YYYYMMDD; reject impossible calendar dates such as 19900230 even when the checksum still happens to compute.
• Region code: digits 1-6 follow GB/T 2260; codes for districts retired after administrative mergers will still pass the checksum but stop resolving to a real district name.
• Legacy 15-digit upgrade: splice '19' between position 6 and position 7, then recompute the MOD 11-2 check digit; this is why a 15-digit number alone has no checksum to verify.
• Gender bit: parity of digit 17, odd is male and even is female, set by the issuing window's sequence allocation and not by self-identification.
• Age boundary: subtract one year when (today's month, day) is earlier than (birth month, day), so a person whose birthday falls later in the year reads one year younger than they will be on their next birthday.