# 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.

Source: https://useqr.app/docs/basics/qr-code-standards-iso-18004 · Last reviewed 2026-08-21 · UseQR is free forever, no signup.

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## What the standard defines

[ISO/IEC 18004](/glossary/iso-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](/docs/spec/data-encoding-modes): numeric, alphanumeric, byte
  and [kanji](/glossary/kanji-mode), and how data maps to bits in each.
- [Error correction](/docs/spec/error-correction-levels-explained): the four levels
  (L 7%, M 15%, Q 25%, H 30%) and the Reed-Solomon mathematics behind them.
- All 40 [versions](/glossary/version), 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](/glossary/mask-pattern) 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](/docs/basics/model-1-vs-model-2-qr), 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](/wifi-qr-code), that `BEGIN:VCARD`
[saves a contact](/vcard-qr-code); 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](/docs/payments/qr-payment-standards-compared)
from EMVCo, NPCI and the European Payments Council. The
[payload](/glossary/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](/upi-qr-code) 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](/glossary/zxing) that serve as de facto
  references. UseQR's own encoder was built against the published structure and verified
  by [round-trip decoding](/docs/developers/why-verify-that-your-qr-code-decodes),
  a conformance check anyone can run at [/validate](/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.

## Try it

- https://useqr.app/url
- https://useqr.app/text
- https://useqr.app/validate