Turn any link into a QR code.

What is inside a QR code, and what makes one fail to scan

A QR code is a grid of modules — the small black and white squares — and the amount of data decides how many there are. The standard runs from version 1 at 21 × 21 modules up to version 40 at 177 × 177. A short link needs a sparse, chunky grid; a long URL with tracking parameters attached needs a dense one. That density is the single biggest factor in whether a code scans easily from across a room or has to be held close under good light.

Every code also carries error correction, at one of four levels: L recovers about 7% of the data, M about 15%, Q about 25% and H about 30%. Higher levels survive a scratched, smudged or partly covered code, which is why anything printed on packaging or a sticker should use Q or H — but the redundancy is stored in extra modules, so raising the level makes the grid denser for the same content. M is the sensible default on screen; H is worth it in print, or if a logo is being placed over the middle.

Print size is where most codes fail in the real world. A workable rule is that the code should be about a tenth of the scanning distance: read from a metre away, print it at roughly 10 cm. Under about 2 cm, dense codes stop resolving on ordinary phone cameras. The quiet zone matters just as much — the specification asks for a clear margin of four modules on every side, and a code butted up against a border or a busy photograph will refuse to scan even when it is large and sharp.

Contrast has one rule that surprises people: it must be dark on light, not the reverse. Scanners look for dark modules against a light background, and a white-on-black code fails on many phones even though it looks correct to a human. Low-contrast colour pairs — mid-grey on beige, two similar pastels — fail for the same reason. If in doubt, black on white always works.

The most expensive mistake is not technical. A QR code generated here is static: the destination is encoded in the pattern itself, so it can never be changed. Print 5,000 flyers pointing at a URL and that URL has to keep working forever, because the code cannot be edited. If the destination might move, point the code at a short link you control and change where that link redirects — the printed pattern stays the same and the destination becomes yours to update.

Shortening the URL before generating is worth doing for another reason: fewer characters means fewer modules, which means a chunkier, more forgiving code at the same print size. Stripping the tracking parameters off a link can be the difference between a version 10 grid and a version 4 one.