Data Redundancy

Data redundancy adds recovery data so originals can be restored from partial loss. QR code error correction uses Reed-Solomon coding: Level L recovers ~7% of the codewords, M ~15%, Q ~25%, H ~30%. Those figures are the recoverable share, not the share taken up by redundancy - at Level H about 66% of all codewords are error correction and only ~34% carry the original data.

Trade-off: more redundancy increases damage resistance but requires larger QR code versions for the same data. Design QR codes with center logos treat covered modules as "damaged" - Level H's ability to recover ~30% of the codewords is what makes a center logo possible at all; that 30% is a share of the data, not of the area, so ~20% coverage is the practical recommendation, with about 25% as the upper guideline under level H error correction.