Picking the Right QR Size for Every Surface
The size of a QR code should be decided by one number first: how far away the scanner will stand. Everything else, from the paper size to the design layout, comes second. The working rule is the 10:1 ratio: the scanning distance should be no more than ten times the width of the code. A business card handed to someone is scanned from about 25 centimeters away, so a 25 millimeter code is the floor, and a little extra is wise. A poster on a wall is scanned from a metre or more, so it needs a code at least 100 to 150 millimeters wide. When you start from distance instead of from the space left over in the layout, the code works the first time.
Each common use case has its own trap. Business cards tempt designers to shrink the code to make room for a name and a logo, but a card scanned at arm's length with a code under 20 millimeters wide will fail on many phones. Flyers and handouts are usually read at 50 to 60 centimeters, so a 50 to 60 millimeter code is the safe target, and it should sit where a thumb does not cover it when the sheet is held. Posters need more than people expect, because viewers step back to read the headline before they approach, and a code sized for close reading will be too small from that first step back. Shop windows add glare and glass into the equation, so a window code should be sized generously and placed where reflections are weakest.
Billboards deserve a special warning. The arithmetic says a billboard scanned from 15 metres needs a code 1.5 metres wide, and from 25 metres it needs 2.5 metres. That is physically possible, but a QR code on a roadside billboard is a bad idea when the audience is driving, because nobody should be scanning while driving. Reserve billboard-sized codes for places where people stand still and look up: transit platforms, stadium concourses, and pedestrian plazas. If the audience is moving in a vehicle, use a short memorable URL or a brand name instead of a code.
Pixels enter the picture when the code goes to print. A code that is 60 millimeters wide is about 2.4 inches, and at 300 DPI it needs roughly 700 pixels across to print crisply. Exporting a 512 pixel image for that job leaves the edges slightly soft, while a 1024 pixel export covers it with room to spare. The quiet zone follows the same arithmetic: it should be at least a tenth of the code width on each side, because it is defined as four modules and modules are roughly a twenty-fifth of the code width. If the margin in your layout is thinner than that, the code is too big for the space, and the fix is a shorter payload or a different placement.
Treat these presets as starting points, then verify with a printed proof. Print the code at the recommended size, mount it where it will live, walk to the real viewing distance, and scan it with a phone in the worst light that spot gets. If the scan is instant, you are done. If it hesitates, go one size up before you blame the design, because size is the cheapest fix in the whole process and it solves more failures than any color or styling change.