QR codes: sizes, errors and best practice
Updated 30 October 2026 · 6 minute read
A QR code is a grid of black and white squares that carries text — usually a web address — in a form a camera can read in milliseconds. It is not an image of a link; it is the link, encoded. That single fact explains most of what follows: there is no server in the middle to break, but also no way to change the destination after you print it.
How much data fits
QR codes come in sizes from version 1 (21 × 21 squares) to version 40 (177 × 177 squares). Capacity depends on the character set and the error-correction level:
| Content | Practical capacity |
|---|---|
| Digits only | up to about 7,000 characters |
| Letters and digits | up to about 4,200 characters |
| Any text (URLs, Urdu, emoji) | up to about 2,900 bytes |
These are theoretical maxima. Every extra character makes the grid denser, and a dense code is harder to scan from a distance or from a cheap camera. For a printed poster, a URL of 30–60 characters is comfortable; for anything near the maximum, the code will need to be large.
Error correction: why part of the code can be damaged
QR codes carry redundant data so that a damaged code can still be read. There are four levels:
- L (7%) — smallest code for a given amount of data, least tolerance.
- M (15%) — the common default; a good balance for screens and clean print.
- Q (25%) — better for stickers, packaging and outdoor use.
- H (30%) — highest tolerance, used when a logo covers the centre.
Our generator uses level M. If you plan to place a logo in the middle or print on a curved surface, regenerate with a higher level in a design tool, or keep the code plain.
How large should it be printed?
A practical rule used in signage is the 10:1 distance rule: the code should be roughly one tenth of the scanning distance. A code scanned from 3 metres should be about 30 cm wide. For a business card read at 20 cm, 2 cm is the minimum and 2.5 cm is comfortable.
- Keep a clear white margin (the “quiet zone”) of at least four modules on every side.
- Use black on white. Dark blue on cream usually works; yellow on white does not.
- Avoid glossy laminate that reflects a phone flash.
- Do not stretch the code — keep it square.
Common mistakes
- Printing too small. The most common failure. Test with a cheap phone at the real distance.
- Low contrast. Inverted codes (white on black) fail on many scanners.
- Cutting the quiet zone. Placing text or a border right against the squares confuses the scanner.
- Linking to a page that moves. A printed code cannot be edited; a short, stable URL is safer.
- Assuming every phone has internet. A code that opens a site is useless offline; for Wi-Fi and contact cards, the data is inside the code itself.
Wi-Fi codes and privacy
A Wi-Fi QR code contains the network name and password in plain text. Anyone who scans it can read the password with a generic scanner app. That is fine for a guest network in a shop; it is not fine for your main home network on a poster by the door. Create a separate guest network for public codes, and change the password when the code is no longer needed.
QR codes and safety
A QR code can point anywhere, including to a phishing page. Before you scan a code you do not recognise:
- preview the link — most camera apps show the URL before opening it;
- be suspicious of codes stuck over other codes, common on parking meters and posters;
- never enter a password or card number on a page reached from a random sticker;
- for payments, prefer the official banking app’s own scanner.
Why local generation is different
Many “free QR” services generate a code that points to their own short link, which then redirects to your URL. That lets them count scans — and lets them switch the destination, or charge you later to keep it working. The code we produce contains your data directly, so it never expires and cannot be redirected. The trade-off is that changing the link means printing a new code.