What is inside a QR code
A QR code is a grid of black and white modules: three large finder squares in the corners, smaller alignment squares, two timing rows, and the data interleaved with Reed–Solomon error-correction codewords and XORed with a mask pattern so the grid never looks like a solid block. The size (version 1 = 21×21 up to version 40 = 177×177) grows with the amount of data; the version and mask chosen for your content are shown next to the code.
Error correction
Level L lets a reader recover from 7% damage, M 15%, Q 25% and H 30%. Higher levels make bigger codes. M is the usual choice; H is for codes that will be printed small, placed on curved surfaces, or have a logo pasted over the middle (which is how branded QR codes work: the logo simply destroys part of the code and the error correction hides it).
Content formats readers understand
| Kind | Payload |
|---|---|
| URL | https://example.com/path |
| Wi-Fi | WIFI:T:WPA;S:MyNetwork;P:secret;H:false;; |
| Contact | BEGIN:VCARD … END:VCARD (vCard 3.0) |
| mailto:[email protected]?subject=… | |
| Phone / SMS | tel:+15550123 · SMSTO:+15550123:text |
| Location | geo:51.4769,0.0005 |
Printing
Use the SVG for anything printed: it scales without blur. Keep the quiet zone (the margin of four modules) clear, keep the contrast high (dark on light; inverted codes fail on many readers), and test with a phone before printing a thousand stickers. A code with a short URL is smaller and scans faster than one with a long one; for a long link, host a redirect on your own domain rather than using a third-party shortener that can expire or track.
Questions
Does this QR code expire or track scans?
No. The code contains your text directly. There is no redirect through a service, nothing to expire and nothing to count scans. If you want analytics, encode a URL on your own domain and log it there.
How much data fits in a QR code?
Up to 2,953 bytes at level L (version 40), or 4,296 alphanumeric or 7,089 numeric characters. Practical codes hold under 300 bytes; a large code needs a large, flat print to scan reliably.
Why is the Wi-Fi code not working?
Check the network type (WPA covers WPA2/WPA3), that the password has no stray spaces, and that the SSID is spelled exactly. iOS and Android both read the WIFI: format natively through the camera app.
Can I add a logo?
Not in this tool, but you can paste one over the centre in any image editor if you generate at level H and keep the logo under about a quarter of the area. Test the result with a reader.