Basics
ISO/IEC 18004 — the QR code standard explained
ISO/IEC 18004 is the international standard defining the QR code symbology - encoding modes, error correction, masking and all forty symbol versions. It has run through four editions, in 2000, 2006, 2015 and 2024. It deliberately says nothing about payloads; URLs, WiFi strings and vCards are conventions layered on top.
What the standard defines
ISO/IEC 18004 specifies the QR code symbology — everything needed for one program to generate a symbol and an unrelated program to decode it identically:
- The four encoding modes — numeric, alphanumeric, byte and kanji — and how data maps to bits in each.
- Error correction: the four levels (L 7%, M 15%, Q 25%, H 30%) and the Reed-Solomon mathematics behind them.
- All 40 versions, from 21 × 21 to 177 × 177 modules, with their capacity tables and alignment-pattern positions.
- The function patterns — finder, timing, alignment — plus format and version information encoding.
- The eight mask patterns and the penalty scoring that selects among them.
- A reference decode algorithm, defining what a conformant reader must recover.
It also defines Micro QR. Its rectangular successor rMQR lives in a separate document, ISO/IEC 23941, and the other 2D symbologies each have their own: Data Matrix (16022), Aztec (24778), PDF417 (15438).
The editions
| Edition | Year | Notable |
|---|---|---|
| 1st | 2000 | First international standardisation; included Model 1 and Model 2 |
| 2nd | 2006 | Written around Model 2, Model 1 relegated to legacy status |
| 3rd | 2015 | Consolidation and corrections |
| 4th | 2024 | Current edition |
The lineage predates ISO: the format was standardised in Japan as JIS X 0510 first, and the Japanese standard remains aligned. For practical purposes every code generated in the last two decades targets the same Model 2 symbology, which is why edition differences never bite ordinary users.
What it deliberately does not define
The standard's payload is bytes, full stop. That a payload beginning https:// opens
a browser, that WIFI:S:...; joins a network, that BEGIN:VCARD
saves a contact — none of this is in ISO/IEC 18004. Those behaviours
are conventions layered on top: URI syntax from the IETF, the WiFi scheme popularised by
scanner apps, vCard from RFC 6350, payment payloads
from EMVCo, NPCI and the European Payments Council. The
payload is a contract between generator and scanning software, and
the standard stays silent on it by design — which is exactly why the same symbology
carries menus in London, UPI payments in Mumbai and boarding passes
everywhere.
Getting it, and whether you need it
ISO sells the standard as a paid PDF through iso.org and national standards bodies — standards development is funded by document sales, so no legal free copy exists. Whether you need one:
- Using QR codes (generating, printing, scanning): no. Nothing in the document changes how you use a generator.
- Implementing a generator or decoder: genuinely useful, but not strictly required — the format is exhaustively documented in public tutorials and, more rigorously, in mature open-source implementations like ZXing that serve as de facto references. UseQR's own encoder was built against the published structure and verified by round-trip decoding — a conformance check anyone can run at /validate.
- Compliance work (regulated labelling, procurement specs citing the standard): yes, buy the current edition; citations need the authoritative text.
FAQ
What is ISO/IEC 18004?
The international standard defining the QR code symbology: encoding modes, error correction, masking, all forty versions and the reference decode algorithm. The current edition is the fourth, published in 2024.
Is the QR code standard free to download?
No. ISO sells it through iso.org and national standards bodies. Implementers commonly work from public documentation and open-source reference implementations instead, which is legal because using the format requires no licence.
Does ISO 18004 define what a QR code contains?
No. It defines how bytes become a scannable pattern and back. Payload meanings — URLs, WiFi credentials, vCards, payment strings — are separate conventions from the IETF, EMVCo, industry practice and scanner vendors.
What standard covers rectangular QR codes?
rMQR has its own document, ISO/IEC 23941, published in 2022. Micro QR is included within ISO/IEC 18004 itself, while Data Matrix, Aztec and PDF417 each have separate ISO standards.
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Related
- Model 1 vs Model 2 QR codes — Model 1 is the original 1994 QR code, capped at version 14 with no alignment patterns. Model 2 extends to version 40 and is the only kind generated today.
- Are QR codes patented and free to use? — Denso Wave holds the original patents but pledged not to enforce them for standard use - and the 1994 patents have expired anyway. The name is trademarked.
- How a QR code is generated, step by step — Encoding runs in eight steps: choose the mode, choose the version, build the bit stream, split into codewords, compute Reed–Solomon error correction,…
- Micro QR codes — the small variant and its big trap — Micro QR shrinks the format to four sizes from 11 x 11 modules with one finder pattern - but most phone cameras cannot read it, which rules out consumers.