Why does a QR code still scan with a logo in the middle?

Question

Weirdo · Accepted Answer

QR codes use Reed-Solomon error correction. ISO/IEC 18004 defines four levels — L, M, Q, H — that can recover about 7%, 15%, 25%, or 30% of codewords. A center logo eats some data modules; if it stays inside that budget, the scanner reconstructs the payload.

ISO/IEC 18004:2024 specifies four Reed-Solomon levels, in rising recovery capacity: L 7%, M 15%, Q 25%, H 30% of codewords. DENSO WAVE, which invented the code, describes the mechanism as Reed-Solomon code added to the original data; factories often pick Q or H, and M (15%) is the usual default. Modules under a logo count as damage. The scanner uses the spare codewords to put them back.

The standard puts finder patterns at three corners so a reader can locate the symbol, measure size, and detect tilt. The center is mostly encoding region. A logo there is covering data and ECC modules, not the corner marks the decoder needs first. Raise the correction level and the symbol grows; if the logo exceeds that level’s recoverable fraction of codewords, the scan fails. Generators that stamp a logo often pick H so the center has budget left.

Why does a QR code still scan with a logo in the middle?

The logo is paid for out of the error-correction budget

Corners locate the code; the middle can take a hit