A QR code can still scan when part of it is missing, smudged, or covered — because every QR code contains more data than the minimum needed to decode the payload. This redundancy is called error correction, and it comes in four levels that trade scan reliability against pattern density.
The four levels
QR codes use Reed-Solomon error correction at one of four levels:
L(Low): up to 7% of the code can be damaged and recovered.M(Medium): up to 15% — the default in most generators.Q(Quartile): up to 25%.H(High): up to 30%.
Higher levels add more redundant data, which requires more cells, which produces a denser pattern. A denser pattern needs more print area or a steadier camera to scan reliably.
Which level to pick
For a URL on a clean digital display, L or M is enough — the code is never physically damaged and scans quickly at low density. For anything printed that will be handled, stuck to surfaces, or exposed to moisture, Q or H is worth the density trade-off.
Why it does not cover every failure
Error correction recovers lost cells, not lost contrast. A code that is too faded, too small, or printed on a reflective surface will fail to scan regardless of the correction level set. Physical size and print contrast matter as much as redundancy.