Error Correction in QR Codes: Why Damaged Codes Still Scan

You may have scanned a QR code on a scratched poster or a crumpled receipt and found that it worked anyway. That is error correction at work.

How it works

QR codes use a method called Reed–Solomon error correction. Extra data is stored alongside the message. If some modules are unreadable, the scanner uses the extra data to rebuild what is missing.

The four levels

Level	Approximate recovery
L (low)	about 7% of the code
M (medium)	about 15%
Q (quartile)	about 25%
H (high)	about 30%

The percentages are the share of the code's data that can be damaged and still recovered.

The trade-off

Higher correction means more extra data, which makes the code larger or denser for the same message.

• Level L suits clean, controlled settings, such as a code on a screen.
• Level M is a common everyday choice.
• Levels Q and H suit codes that will be handled, outdoors or on packaging.

What damage it survives

• small scratches
• smudges and dirt
• part of the code covered by a small sticker
• wear on a printed label

What it does not survive

Error correction cannot save a code that is missing a finder pattern, is badly out of focus, or has lost too much of its area. The three corner squares must stay readable.

Tips

• Protect the corners of any printed code.
• Choose a higher level for stickers and outdoor signs.
• If a code will not scan, move closer, improve the light, or try a slightly different angle.

QR Code Scanner is built to scan quickly, and its scan history keeps a record of what you have read, so you can find a code's result again.

Scan and create QR codes and barcodes with QR Code Scanner →