ISO 11784 and 11785 Explained: How the 15-Digit Animal ID Number Is Built

Every ISO animal tag, from a 2 × 12 mm glass microchip to a cattle ear tag, stores the same kind of number. Two standards govern it. ISO 11784 defines the structure of the code, and ISO 11785 defines how a reader gets it out of the tag.

Most people only ever see the result: a 15-digit number such as 276 000123456789. This guide takes that number apart. It shows which bits sit behind it, what the first three digits mean, how the code crosses the air interface, and what to check so the number reaches your software intact.

Two standards, two jobs

The 1996 edition of ISO 11784 was amended in 2004 and 2010 to add fields to the code. The current third edition, published in December 2024, consolidates those amendments, and the layout below follows that amended structure.

A tag can meet ISO 11784 and still use either protocol from ISO 11785, so a reader must support the right one. FDX-B vs HDX animal tags covers that difference in detail.

The 64-bit code, bit by bit

The 15-digit number uses only the last two fields, 48 of the 64 bits. The flags, retagging counter and user information are usually not shown on screen or sent in a reader’s default output. If your application needs them, for example to record retagging, ask whether the reader can output the full code.

The field sizes explain the number of digits. Ten bits give values up to 1023, which fit in three decimal digits. Thirty-eight bits give values up to 274,877,906,943, which is twelve digits. Three plus twelve makes fifteen.

Worked example: from bits to 15 digits

Take a tag coded with the ISO 3166 country code for Germany (276) and the national ID 123456789. The numbers are illustrative.

Two details catch people out:

• Zero padding. The national ID is always printed as 12 digits, so 123456789 becomes 000123456789. Country codes are padded to three digits in the same way: Australia’s 36 is written 036.
• Grouping. Readers, registries and printed tags may show the same ID as 276000123456789, 276 000123456789 or with a dash. The digits are what count; the separator is only formatting.

The number printed on a livestock ear tag is set by the national scheme and can look different from the electronic ID, for example with country letters or herd and animal numbers. Check how your scheme maps one to the other before matching printed and electronic numbers.

Country codes and manufacturer codes

The first three digits tell you who is responsible for keeping the ID unique.

ISO appointed ICAR as the registration authority for manufacturer codes, and ICAR also keeps a registry of devices tested for conformance with ISO 11784/11785, including readers. Under ICAR’s guidelines, a country code may be used only where that country’s competent authority has authorized it and takes responsibility for the numbering, typically for a given species. Otherwise a manufacturer code is used.

That is why many pet microchips begin with a 9: they carry a manufacturer code rather than a country code. Livestock tags in national programs more often carry the country code; our livestock EID ear tag readers page covers reading them on the farm. Neither is more “ISO” than the other.

How ISO 11785 carries the code

The same 64 bits travel differently depending on the protocol:

In both protocols a 16-bit CRC (CCITT) covers the 64 ID bits. The reader checks it, so a damaged reply is normally rejected rather than shown as a wrong number. The 24-bit extension field carries extra data when the data block flag is set.

In the FDX-B telegram the national ID is sent first, least significant bit first, followed by the country code and the flags. This is why raw hexadecimal dumps from different readers can look unrelated even for the same tag.

What the reader sends to your software

The tag always holds the same 64 bits. What your software receives depends on the reader’s output setting:

For our 134.2kHz animal ID readers:

• The A110-U and A140-U desktop readers type the tag number at the cursor with no driver. The exact string is confirmed in your quotation, so tell us which format your software stores.
• The A220-C USB-C microchip reader for Android types a 10-digit decimal (4 bytes) by default. That cannot carry a full ISO ID, so if you need the 15-digit number, ask for a custom output format; the format is confirmed in your quotation.
• The handhelds show the ID on their OLED. The A520-M and A540-M upload reads live or from memory, and the A530-U uploads stored records over USB. The A660-M 134.2kHz EID panel reader sends the ID over RS232 or RS485. For all of them, the upload or serial format is confirmed in your quotation.

All of them read ISO FDX-B tags and 125kHz EMID tags, and none of them writes tags. EMID tags do not follow ISO 11784 and have no country code, so they come out as a shorter number. If you handle both, your database has to accept both lengths.

Checklist: getting ISO animal IDs into your system

1. Store the ID as text, 15 characters for an ISO ID, not as a number. Number fields drop the leading zero of codes such as 036, and spreadsheets show long numbers as 2.76E+14. The Excel guide covers setting a column to text.
2. Choose one display format (with or without a space after the first three digits) and strip separators before comparing IDs.
3. Classify the first three digits on import: country code, shared code 900, other manufacturer code, or 999 test tag. Reject 999 in production records.
4. Decide whether you need the flags. If retagging or species information matters, confirm the reader can output the full 64-bit code; most readers show only the 15 digits.
5. Confirm the protocol. Make sure the reader supports FDX-B, HDX or both, to match your tags.
6. Plan for non-ISO tags. EMID and older 125kHz formats give shorter numbers or no read at all on an ISO-only reader.
7. Get a sample output string from the reader before you write import code, and test it with tags from your own supply.
8. Ask about certification if a tender requires it. Do not assume a reader is ICAR-tested unless you receive the test certificate.

To check a reader against your software, order samples or send your tag type and the string your system expects with your inquiry. We will confirm the output format in your quotation.