QR Code Decoder

How QR code decoding works

A QR code is a 2D barcode invented by Denso Wave in 1994 to track car parts on a factory floor. The black-and-white squares are not random — they encode digital data using a strict spec (ISO/IEC 18004) covering character encoding, error correction, masking, and the three large "finder" squares in the corners that let a scanner orient the image even when it is rotated or skewed.

To decode (sometimes called decipher) a QR code, software reads the image, finds the three finder patterns, normalises the orientation, samples the grid, decodes the bit stream, applies Reed-Solomon error correction (which is why a QR code still scans with up to ~30% of its surface damaged or covered), and returns the embedded text. This tool does all of that locally in your browser using the open-source jsQR library.

What can a QR code actually contain?

QR codes just hold a string of text. The convention is that scanner apps recognise certain prefixes and offer to do something useful with them. The decoded payload here is the raw string — you can read it directly. Common formats:

• • URL: https://example.com — most QR codes you see on posters and menus.
• • Wi-Fi: WIFI:T:WPA;S:NetworkName;P:password;; — the encryption type, SSID, and password.
• • vCard contact: a multi-line block starting BEGIN:VCARD with name, phone, email, organisation.
• • Email: mailto:you@example.com?subject=Hello.
• • Phone / SMS: tel:+1234567890 or sms:+1234567890.
• • Calendar event: a BEGIN:VEVENT block (iCal format).
• • Geolocation: geo:37.7749,-122.4194.
• • Plain text: anything else — a note, a coupon code, a serial number.

Why decode QR codes on your computer instead of scanning with your phone?

Safety. Phone camera apps often auto-open the embedded URL. If a QR code on a poster or email is malicious, decoding it on your computer first lets you read the URL before deciding to open it. Treat QR codes from unknown sources the way you would treat shortened links — decode and inspect first.

Practicality. If the QR code is already on your screen (in an email, a PDF, a screenshot a friend sent), opening your phone camera to scan your monitor is awkward and often fails on reflective screens. Uploading a screenshot here is faster.

Privacy. Many free QR scanner apps upload your image to their servers, run analytics, or insert ads. This page does not — the image never leaves your device.

Going the other direction

Going the other direction? Use the QR code generator to create your own QR codes for URLs, text, email, or phone. For Wi-Fi codes specifically, the Wi-Fi QR generator builds a code that lets guests join a network without typing the password. For standard 1D barcodes (EAN, UPC, Code 128), see the barcode generator.

The QR Code specification is published by ISO/IEC as ISO/IEC 18004. The Wikipedia QR code article has a thorough technical breakdown of encoding modes, error correction levels, and version sizes if you want to go deeper.