How Big Does It Have to Be?

A QR code reads from about ten times its own width. Two centimetres for a phone held at twenty. Ten centimetres for a shelf sign at a metre. Fifty centimetres for a poster seen from five.

That ten to one figure is a fine first guess. It is also not in the standard, and the thing underneath it is what decides whether your code scans.

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The standard is ISO 18004. It defines how a QR symbol is built, its versions, its encoding and its error correction. It sets no ratio of code width to reading distance at all. Denso Wave, the company that invented the QR code, does not state the ten to one rule in its own documentation either.

The rule survives because it is convenient and roughly right. For a typical phone camera, dividing the distance by ten happens to land you above the floor that does matter. Treat it as a shortcut, then check it against the thing that is real.

What sets the true minimum is resolution. Scanner makers put the decode floor for a two dimensional code at about two and a half pixels per module, and at one pixel per module for an ordinary barcode. A module is one black or white cell in the grid. Design for three to four pixels per module, because that is your margin for motion blur and a cheap camera.

For print, the smallest module most people trust sits near four tenths of a millimetre. Three tenths is aggressive and wants a good printer. Denso Wave gives you the formula outright. Millimetres per module equals twenty five point four divided by your printer DPI, multiplied by the dots you allocate to each module. Their own worked example, four hundred DPI at four dots per module, comes out around a quarter of a millimetre.

Then the geometry, which is where this catches people out.

A QR grid has seventeen plus four times the version number, per side. Version one is twenty one by twenty one. Version forty is a hundred and seventy seven by a hundred and seventy seven. So the more you encode, the more modules you need, and at a fixed module size the printed square gets physically larger.

Which means putting a long address with a full set of tracking parameters into a code is a size decision, whether or not anybody tells you that at the time. Shorten the link and the code gets smaller and easier to read. That is the practical argument for a short link on print, and it has nothing to do with branding.

The quiet zone is not optional either. Four modules of blank margin on all four sides. That one is in the standard, and designers trim it constantly, because on a layout it looks like padding.

One more if you print retail barcodes. EAN thirteen and UPC A run at eighty to two hundred percent magnification, which is a quarter of a millimetre to two thirds of a millimetre per bar. Eighty percent is only valid for thermal or laser printing.

Three habits, then. Export SVG rather than a PNG, so the code stays sharp at whatever size the layout ends up at. Keep the quiet zone. And test-scan a printed copy, at the real distance, in the light it will live in, before you commit the full run.

When we built the size calculator on our own site, the maths we put behind it is Denso Wave's, and not the ten to one rule. The rule gets you a number. The formula tells you whether the number survives contact with a printer.

I am Ankur, and I built Whooshly, a campaign link toolkit you pay for once instead of by the month. Short links, QR codes, UTMs. If you want the code you printed to still work next year, we are at whooshly.co.