Quiet zone
The blank margin around a QR code — four modules minimum, per the spec.
The quiet zone is the clear border surrounding a QR code. ISO/IEC 18004 requires it to be at least four modules wide on all four sides, where a module is one of the small squares the code is built from.
Why it exists. A scanner locates a code by finding its finder patterns against a plain background. Without the margin, surrounding artwork, text or a border can be mistaken for part of the code, and detection fails before decoding is even attempted.
How it gets broken. Almost always in design. A code is dropped into a tight layout, cropped to fit a box, or given a decorative frame flush against the modules. It usually still scans on the designer's screen at 400px, then fails on a printed card at 20mm.
How to check. Measure the blank border and divide by the module size. If the margin is less than four modules, add space. On a 25×25-module code printed at 25mm, each module is 1mm, so the quiet zone needs to be 4mm.
In practice
A café orders table tents with a 25mm code in a decorative border that sits flush against the pattern. The code is 25×25 modules, so each module is 1mm and the required quiet zone is 4mm. It scans on the designer's monitor and fails on roughly half the phones in the room, because the border is being read as part of the code.
The fix costs nothing: 4mm of white between the code and anything else. The calculation is always the same — printed width divided by module count, multiplied by four.
Questions
Does the quiet zone have to be white?
It has to be the same light colour as the code's background and uniform. A pale tint is fine; a pattern, photograph or gradient is not.
Can I put a frame around the code?
Yes, outside the quiet zone. Call-to-action frames work well when the four-module margin sits between the code and the frame.