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Two numbers belong on every print spec you hand a shop: a quiet zone of at least 4 modules on all four sides, and a printed size of at least about 20–25 mm (roughly 1 inch) square. Both are about whether a phone camera can decode the code at all — and both fail silently, producing a piece that looks perfect and does not scan.

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The quiet zone: 4 modules, and nothing may cross it

A module is one of the small squares the code is built from. The quiet zone is blank space on all four sides, at least 4 modules wide, and the scanner uses it to find the code’s edges. Nothing can cross into it. Not a logo, not a border, not a line of text, not a mounting hole, not the edge of a laminate or an overlay film. Any of those can stop the code decoding while leaving artwork that looks entirely correct to the eye. Mark the quiet zone on your spec as a boundary, not an afterthought — the shop needs to know where the clear space ends, not only where the code sits. This matters most on small tags, where a fixing hole and the code are competing for the same few millimeters.

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The exported margin is the quiet zone — protect it

Every code QRtub exports carries a 4-module margin, SVG and PNG alike, which is what print specs ask for. So a placed file arrives with its quiet zone intact. What the file cannot do is defend it. A rule, a border, a fixing hole or a trim edge that sits inside that margin takes the clear space back, and the code still looks correct. Mark the boundary on your spec rather than assuming the built-in margin survives layout.

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Minimum size: about 20–25 mm

As a rule of thumb, do not print a code smaller than about 20–25 mm square if it needs to be reliably scanned by a phone camera. Below that, the modules get small enough that camera focus, print resolution, and material texture start to matter more than the code itself. Scanning distance pushes that number up, not down. A code on a plant room door read from two meters away needs to be much larger than one on a handheld tool.

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Size, format, and what error correction can and cannot do

QRtub exports SVG or PNG, black on white, with the error-correction level of your choice. Two practical consequences for a print spec. Use SVG for anything printed. It is vector, so one file covers a 20 mm tool tag and a 400 mm door sign with identical edges. PNG is a fixed 1024 × 1024 pixels — roughly 87 mm at 300 dpi, which is comfortable for a tag or a sticker, tight for a plaque, and not enough for a sign. Enlarged well past that, a PNG’s module edges soften enough to stop scanning. If a shop only has the PNG and needs it larger, give them the Full URL column from your print list and let them regenerate at press size — that is normal practice, not a workaround. Error correction is a real lever now, but it is not a substitute for size. For a piece that will live outdoors, be laminated, or get scratched, raise the level — Q for most printed tags and H where the code will be damaged or partly covered. For QRtub’s short addresses the step up is often free, so this usually costs nothing; see QR Code Standards. What it cannot do is rescue a code that was printed too small. A pristine 10 mm code at H still will not scan reliably, because the failure there is the camera resolving individual modules, not missing data. Size first, then error correction — and print the address in readable text nearby either way, so someone can type it when the code is beyond saving.