Making QR Codes That Actually Scan

Error correction: how much damage it survives

QR codes carry redundant data so they still read when part of the pattern is dirty, torn, curved or covered. There are four levels, and choosing one is a direct trade between robustness and density.

Higher correction is not free: the redundant data occupies modules, so the same URL at level H produces a denser grid than at level M. The common mistake is choosing H because it sounds safest and then printing the code at the size that suited level M, which makes each module smaller and undoes the benefit. Pick the level for the conditions, then size the code for the level.

Level H is what makes the logo-in-the-middle style possible. If up to 30% of the modules can be lost, a logo covering the centre is recoverable — provided it stays in the centre and does not touch any of the three corner finder patterns. Cover a corner and no amount of error correction helps, because the scanner can no longer locate the code at all.

Size and the ten-to-one rule

The industry convention is simple arithmetic: the code's side length should be at least a tenth of the intended scanning distance. Scanning from two metres away needs a code 20 cm across. Scanning from 30 cm — a table card, a leaflet in the hand — needs about 3 cm. Below roughly 2 × 2 cm, even a close-range code becomes unreliable on cheaper cameras.

Two adjustments to the arithmetic. Add margin if the code is dense — a version 10 code at the minimum size for its distance has very small individual modules, and print registration or a slightly out-of-focus camera will defeat it. And subtract nothing for a screen: a code displayed on a phone for someone else to scan is at close range but is also glossy and reflective, so keep it generous and turn the brightness up.

The quiet zone nobody leaves

The specification requires a clear margin of at least four modules on every side — the quiet zone. It is not decoration. The scanner uses that blank border to find the boundary of the code, and without it the pattern blends into whatever surrounds it and detection fails or becomes intermittent.

This is the single most common failure in real-world design work, because a designer places the code neatly against a coloured panel, a photograph or a line of text, and it looks better that way. Four modules is roughly 10% of the code's width, so on a 3 cm code that is about 3 mm of clear space on all four sides. Generators usually include it in the exported image; cropping the export tightly to save space removes it again.

Printing and material problems

• Ink spread. On absorbent stock, printed black modules grow slightly and can bridge to their neighbours. Print a proof on the real material rather than on office paper.
• Glossy laminate. Reflections from overhead lighting wash out part of the code. Matt finishes scan considerably more reliably.
• Curved surfaces. Bottles, cups and tubes distort the grid. Use level Q or H, keep the code small relative to the curve, and test on the actual object.
• Vector, not raster. Export the code as SVG or PDF for print where you can. A JPEG of a QR code carries compression artefacts on exactly the hard black-and-white edges the scanner is measuring.
• Never resize a raster code by dragging. Non-integer scaling makes some modules a pixel wider than others, which is visible to a scanner even when it is not to you. Regenerate at the size you need.

Before anything goes to print in quantity, decode the final artwork rather than the original export. The QR reader reads a code out of an image file. To check a print PDF, turn the page into an image with PDF to image first, then decode that — the artefact that will actually be printed — rather than the file you generated an hour and several design steps ago.

Static codes, dynamic codes and what you are agreeing to

A QR code encodes text and nothing else. When it contains a URL, that URL is fixed forever — printed on ten thousand leaflets, it cannot be changed. This is what dynamic QR services solve: they encode a link to their own domain, which redirects to your real destination, so you can change the target later and count the scans.

The trade is worth stating plainly. Your printed material now depends on a third party's domain continuing to exist and continuing to redirect, and every scan passes through their analytics. If that service shuts down or changes its pricing, every code you have printed becomes a dead link. A static code pointing at a URL you control has none of those risks, and you can still get analytics by pointing it at a page on your own site that redirects.

One note on generation. A QR code frequently encodes something private — a Wi-Fi password, a personal contact card, a private link — and most online generators create the code on their server, which means they have seen it. The generator here builds the code entirely on your device, so the contents are never sent to our servers, and the reader likewise decodes images locally rather than uploading them.