# QR code guides > Free QR code guides: how the format works, what size to print, error correction levels explained, why codes fail to scan, and whether QR codes are safe. Source: https://qr-code-build.com/learn/ Last reviewed: 2026-08-24 Publisher: QR Code Builder ## Written from production, not from other websites Most QR code advice online is copied between sites, and the copying propagates errors. Figures for capacity disagree with the specification. Minimum sizes get quoted with no reference to module size, which is the number that actually decides whether a code scans. Scan statistics circulate with no traceable source. These guides take a different approach. Technical claims are checked against ISO/IEC 18004 or derived by testing a reference encoder, and where something depends on a device or a press we say so rather than giving a false single answer. Where we do not know, the page says that too. They are written for people who have to make something work in the physical world — a menu that scans in dim restaurant lighting, a label that survives a warehouse, a board read from across a garden. ## Three routes through the material - **Printing something soon?** Read the sizing guide first, then error correction. Those two cover most failures before they happen. - **Something already broken?** Go to the failure diagnosis guide. The causes are ordered by how often each turns out to be responsible. - **Want to understand the format?** Start with what a QR code is, then how they work, which covers masking and Reed–Solomon correction. - **Worried about safety?** The quishing guide explains what a code can and cannot do to a phone, and how to check one in ten seconds. ## The fundamentals What is a QR code? What the format is, where it came from, and what it can hold. How QR codes work Finder patterns, masking, Reed–Solomon and the anatomy of the grid. QR vs barcode vs NFC Which technology fits which job, with the trade-offs. ## Getting it onto paper correctly What size should a QR code be? The distance rule, module size floors and a size table by use. Error correction levels What L, M, Q and H actually do, and which to choose. Why QR codes fail to scan Twelve causes in order of frequency, each with a fix. ## Safety, permanence and reference data Are QR codes safe? Quishing, what a code can and cannot do, and how to check one. Do QR codes expire? Why some codes stop working and static ones never do. Specifications and reference data Full capacity tables for all 40 versions and 4 correction levels. ## Build something - [Link QR code generatorTool](/url-qr-code-generator/) - [Wi-Fi QR code generatorTool](/wifi-qr-code-generator/) - [vCard QR code generatorTool](/vcard-qr-code-generator/) - [Read a QR code from an imageTool](/qr-code-scanner/) - [Bulk generatorTool](/bulk-qr-code-generator/) - [Custom designTool](/custom-qr-code/) ## Frequently asked questions ### Are these guides free to read? Yes, all of them, with no sign-up and no email capture. They can be quoted with attribution. ### How often are the guides updated? Each carries a last-reviewed date, and the review is real: figures, links and platform behaviour are re-checked rather than the date simply being changed. ### Which guide should I read before a print run? The sizing guide and the error correction guide. Between them they prevent most print failures, which almost always come down to module size or the wrong correction level. ### Can I cite this material? Yes. The reference data page in particular is published for citation, with figures derived from a reference encoder and cross-checked against ISO/IEC 18004. --- Licence: this page may be quoted with attribution to QR Code Builder (https://qr-code-build.com/learn/).