QR code size and scan distance: the numbers, not the guesses

Every conversation about QR size ends up in the wrong units. People ask how many centimetres the code should be. The scanner does not measure centimetres — it measures modules, the small squares the symbol is built from, and it needs each one to land on enough camera pixels to be resolved.

Get the module size right and the centimetres follow.

The rule that actually works

Width of the code is about one tenth of the distance it will be scanned from. It is a rule of thumb rather than a clause in the standard, and it holds up because it encodes something real: a phone camera at distance D resolves roughly a fixed angular size, and a ten-to-one ratio keeps a typical symbol's modules above that threshold.

Where it lives	Scanned from	Minimum width	Comfortable width
Business card, receipt	20 cm	2 cm	2.5 cm
Table tent, menu, packaging	40 cm	4 cm	5 cm
A4 poster in a lift	1 m	10 cm	12 cm
Shop window, seen from the pavement	3 m	30 cm	35 cm
Trade show banner	5 m	50 cm	60 cm

The comfortable column is what to actually print. The minimum column assumes a good camera, even lighting and a clean print, and real environments rarely give you all three.

Why the payload changes the physical size

A QR symbol comes in versions. Version 1 is 21 modules a side, and each version adds four modules. The version is chosen by how much data you put in and how much error correction you ask for.

Version	Modules a side	Characters at level M
1	21	14
2	25	26
3	29	42
4	33	62

A gc.mw QR encodes https://gc.mw/abcd?qr — 21 characters, including the marker that lets us count scans separately from clicks. That fits version 2 at level M: 25 modules a side.

A typical campaign URL with UTM tags is 60 to 120 characters, which is version 4 or 5. Print both at 3 cm and the short link gives you 1.2 mm modules while the long URL gives you 0.8 mm or less. In a shop with fluorescent lighting and a two-year-old phone, that difference is the entire scan rate.

Error correction, and the trap inside it

Four levels exist: L recovers about 7 per cent of the symbol, M about 15, Q about 25, H about 30. Higher levels survive more damage — a scratch, a fold, a coffee ring, a logo.

The trap is that error correction data occupies the same symbol. Raise the level and the same content needs a higher version, so at a fixed print size every module shrinks. A code at level H printed small can be measurably harder to scan than the same content at level M, because you traded resolution for redundancy you did not need.

Our default is level M, which is the right answer for almost every printed link. It goes to H automatically when a centre logo is placed, because then you genuinely are destroying a quarter of the symbol on purpose.

Testing it properly

1. 1

   Print at final size on final stock

   A code on office paper and a code on matte laminate behave differently. Laminate reflects; uncoated stock spreads ink. Test what you will actually produce.

2. 2

   Use the worst phone in the office

   A three-year-old mid-range Android with a smudged lens is the median scanner, not your new iPhone.

3. 3

   Stand where the customer stands

   Not leaning over the desk. If it is a window decal, go outside on a sunny day and try it from the pavement, at an angle, with the reflection.

4. 4

   Check the quiet zone survived

   Layout tools crop. Confirm four modules of white on every side in the final PDF, not in the source file.

When to stop using a QR code

If the reader is more than about 5 m away, or moving, or the code will be printed under 1.5 cm, a QR code is the wrong tool. Print a short link people can read and type instead — gc.mw/menu is ten characters and works from a moving bus, which no QR code does.