QR code size calculator

Two independent constraints set the size, and the code has to satisfy both. The first is the reader: a phone camera resolves a QR code reliably when the code is about one tenth of the distance it is scanned from, so the distance floor is simply the scan distance divided by ten. The second is the print process: a module, one of the small squares, needs about 0.4 mm of ink in offset or digital print and about 0.6 mm on a thermal label or a rough surface, so the module floor is the module count times that figure. The print-size guide explains where both rules come from; the calculator only does the arithmetic.

The module count is not something you choose. It follows from the version the encoder picks for your content, and a version has 17 + 4 × version modules per side: version 1 is 21, version 10 is 57. https://example.com/menu is 24 bytes and becomes a version 2 symbol at level M, 25 × 25 modules.

Put that on a poster read from 1 m. The distance floor is 10 cm. The module floor is 25 × 0.4 mm = 1 cm, or 1.32 cm once the four-module quiet zone on each side is added. The distance wins by a wide margin, so the recommendation is 10 cm plus 25% headroom: 12.5 cm. At that size every module is 5 mm and the quiet zone is 20 mm on every side.

Now the other case. The same link with four tracking parameters is 96 bytes, version 6, 41 × 41 modules, printed on a thermal label read from 20 cm. The distance floor is 2 cm, but the module floor is 41 × 0.6 mm = 2.46 cm — larger, so the module count is the binding constraint and the recommendation is 3.5 cm. The distance rule would have let you print it smaller, and it would not have scanned.

Every character you add has to fit somewhere, and when the current version is full the encoder moves to the next one, which adds four modules per side. The example link is 25 modules across; the tracked version of the same link is 41. At a fixed 2 cm on a business card that is the difference between 0.8 mm modules and 0.49 mm modules — the second is barely above the 0.4 mm floor for paper and below the 0.6 mm a thermal printer needs.

Error correction pulls the same lever from the other side. Raising the level adds redundant codewords, which fills the version sooner, which adds modules: the calculator shows the module count for all four levels under the buttons so you can see when a step up is free and when it costs a version. What L, M, Q and H really cost you works through that trade-off in millimetres. If a code that should be large enough still fails, the not-scanning checklist covers the other causes — contrast, quiet zone, glare — and how to test a QR code describes the print test worth doing before a run. A logo in the middle is the one thing the calculator does not model; see the last question below.