Reference
How QR codes actually work.
Enough of the mechanics to make good decisions about size, error correction and print quality — without a lecture on Reed–Solomon mathematics.
The mechanics
Versions and module counts
A QR code is a grid of black and white squares called modules. The grid size is set by the version, from 1 to 40, and follows a fixed formula: modules = 4 × version + 17.
| Version | Grid | Roughly holds |
|---|---|---|
| 1 | 21 × 21 | 25 characters |
| 3 | 29 × 29 | 70 characters |
| 10 | 57 × 57 | 395 characters |
| 40 | 177 × 177 | 4,296 characters |
The generator on this site picks the smallest version your data fits into, and reports it under the preview.
Error correction
Redundant data is woven through the code so it survives damage. Four levels trade capacity for resilience:
| Level | Recovers | Use it for |
|---|---|---|
| L | ~7% | Clean screens, very long data |
| M | ~15% | General printing — a sensible default |
| Q | ~25% | Business cards, packaging |
| H | ~30% | Stickers, outdoor signage, codes with a logo |
Level H is what makes a centre logo possible: the logo covers modules, and the error correction reconstructs them.
The quiet zone
The blank border around a code is structural, not decorative. The specification calls for 4 modules of clear space. Designers crop it to make a layout tidier, and it is the single most common reason a professionally printed code fails to scan.
How big to print
The working rule is 10:1 — minimum width equals scanning distance divided by ten.
| Read from | Minimum width | Typical use |
|---|---|---|
| 20 cm | 2 cm | Business card |
| 30 cm | 3 cm | Table card, menu |
| 1 m | 10 cm | Counter sign, flyer |
| 3 m | 30 cm | A-frame, window decal |
| 10 m | 1 m | Billboard, building sign |
Add margin on top of the minimum. Denser codes, poor lighting and older phone cameras all push the real requirement upwards.
Contrast and colour
- Dark modules on a light background is what scanners expect. Inverting it fails on many devices.
- Keep strong contrast between the two. Pale grey on white is a common and fatal design choice.
- Never place a code over a photo or a busy gradient.
- Print with vector SVG. A stretched PNG blurs module edges and scanning degrades.
Straight answers
Common questions
How much data fits in a QR code?
The largest version (40) holds about 4,296 alphanumeric characters at the lowest error correction. In practice, keep links under about 100 characters — anything longer produces a dense code that needs to be printed much larger.
What do the three big squares do?
They are finder patterns. They let a scanner locate the code and work out its rotation, which is why a QR code reads upside down or at an angle. The smaller square near the fourth corner is an alignment pattern that corrects for perspective.
Why does a QR code still scan when it is damaged?
Reed–Solomon error correction stores redundant data throughout the code. At level H, roughly 30% of the modules can be missing or obscured and the original data is still recoverable.
Can QR codes contain a virus?
The code itself is only text — it cannot run anything. The risk is where it sends you. Treat an unexpected code in a car park or on a parking meter the same way you would treat an unexpected link in a text message, and check the address before tapping through.
Do QR codes work without internet?
Scanning always works offline, because the data is in the pattern. Whether anything useful happens depends on the content: WiFi and contact codes work with no connection, while a link needs the internet to load the page.