How big should a printed QR code be?
Everyone asks this right before sending artwork to the printer, and the short answer is that it depends on one thing only: the distance people will scan it from. Everything else is detail, but three of those details ruin codes daily and are worth knowing before you print a thousand units.
The 10 to 1 rule
The code should measure roughly one tenth of the distance it will be scanned from. If somebody reads it with the phone 30 centimetres away, as on a menu or a card, 3 centimetres a side is enough. On a poster read from 2 metres you need 20 centimetres. For window graphics scanned from the pavement, 5 metres out, you need 50 centimetres.
It is a rough and deliberately conservative rule, which is exactly why it works. Modern phones usually read from further away than it predicts, but that extra margin is what covers you when the light is poor, the camera is old, or the person's hand is not steady.
The absolute minimum: 2 centimetres
Below 2 by 2 centimetres a QR code starts failing even at close range. The reason is not the camera but the printing: each little square needs enough ink dots to stay defined, and when the code is very small the squares bleed into each other.
That minimum climbs if the content is long. A code holding 30 characters has few large squares; the same patch of paper holding 300 characters has many tiny ones. Which is why the first fix for a small code that will not scan is not a design change: it is shortening what the code stores.
The white margin almost everyone forgets
A QR code needs an empty border around it so the reader can tell where it starts and ends. It is called the quiet zone and it has to be at least four modules wide, where a module is one of the code's small squares. In practice: leave a white margin around it about 10 per cent of the code's width.
This is the most common mistake in artwork made by someone who does not know the detail. The code looks lovely tucked right against the edge of the card, or with text pressed up beside it, and suddenly it scans badly for no apparent reason. If you have a code that fails and you cannot work out why, check the margin before anything else.
Contrast, and why not to invert it
You need strong contrast between the squares and the background. Dark on light is what readers expect, and black on white is still the most reliable. Coloured codes work if the colour is dark and saturated and the background is light.
What tends to fail is the reverse: light squares on a dark background. Plenty of modern readers handle it, but not all of them, and on a printed piece that will sit on a wall for five years it is not worth gambling for the sake of style. Putting the code over a photo is a bad idea too: even where the contrast looks fine, the variation in the background confuses the reader.
Surfaces and materials
Matte paper scans better than gloss, because glare throws reflections over part of the code precisely when someone brings the phone closer. If your piece is laminated or sits behind glass, size the code about 20 per cent above what the 10 to 1 rule dictates: the extra margin absorbs the reflections.
Curved surfaces are the hardest case. On a bottle, a can or a tube the code distorts and part of it never faces the camera flat. The practical answer is printing it larger than seems necessary and placing it on the flattest area available. And testing it on the real container before the print run, not on the design on screen.