What the quiet zone does
A camera locates a QR code by finding the three large corner squares against a plain background. The quiet zone is the blank border that gives those squares a clean edge. Without it, a scanner may read a neighbouring line of text or a border as part of the code and give up.
How wide it should be
The standard asks for a margin four squares wide on every side, where a square is one of the small dots in the pattern. In practice, a margin of roughly a tenth of the code's width on each side is a safe way to think about it. Downloads from the generator include the margin, so a downloaded file is ready to place as it is.
How designs lose the margin
- Cropping the image to the edge of the dots, usually to make the code look bigger in the layout.
- A border or box drawn tight around the code. A frame is fine when it sits outside the margin, which is how the frames in the generator work.
- Text or icons pushed up against the code, such as "Scan me" placed too close.
- Busy backgrounds. A photo or pattern behind the code gives the camera nothing plain to find. Put the code on a white panel.
- Dark backgrounds with a transparent code. The margin becomes dark too, so the corner squares have no edge. Use a light panel behind the code.
A quick check
Look at the final artwork and ask whether anything other than plain background touches the code on any side. If it does, move it. Then scan a print at the real size. A code that loses its margin often still scans on a bright screen and fails on paper.
Sources
The four-module margin is part of the QR code specification (ISO/IEC 18004). The official QR code reference site puts it plainly: "QR Code requires a four-module wide margin at all sides of a symbol": Point for determining the code area, QRcode.com.