QR Code to Text — Extract the Payload From Any QR Image

What a QR code actually holds

A QR symbol stores a string of bytes, and nothing in the symbol says what those bytes mean. Everything a phone does on a scan comes from a prefix in the decoded text: https:// opens a browser, WIFI: offers to join a network, BEGIN:VCARD offers to save a contact, tel: dials, mailto: composes, otpauth:// adds a two-factor secret. Reading the raw string is the only way to see which of those you are being handed.

Capacity depends on the symbol version and the encoding mode. Version 1 is a 21 x 21 grid of modules, each version adds four modules per side, and version 40 reaches 177 x 177. Numeric mode packs digits tightly, alphanumeric mode covers digits, uppercase A to Z and a handful of punctuation characters, and byte mode carries arbitrary data, which is what a URL uses. At the largest version and the lightest error correction the ceiling is roughly seven thousand digits or a little under three thousand bytes. Real codes are far smaller, because a dense symbol needs a bigger print and a better camera.

Error correction, and why a damaged code still reads

Every QR symbol carries Reed-Solomon error correction, which is why one with a logo stamped over the middle still scans. The level is chosen when the code is made, not when it is read: the decoder recovers it from the format information around the finder patterns.

When a code will not decode

• The quiet zone has been cropped. A QR symbol needs a blank margin around the whole pattern, and a tight crop removes exactly the reference the decoder needs.
• The photo was taken at an angle. Perspective distortion is correctable up to a point, but shooting square to the code is faster than fighting it.
• The code is too small in frame. Fill more of the picture with the symbol so each module lands on several pixels.
• Contrast is inverted or washed out. Light modules on a dark ground read on some decoders and not others; glare from a glossy print defeats all of them.

For the other direction, the QR code generator writes the same payload formats. Linear retail codes are a different symbology family and belong to the barcode reader.