QR Code Error Correction Levels L, M, Q and H Explained

A QR code stores your content as a sequence of 8-bit codewords. Alongside those data codewords, the generator adds error correction codewords calculated with Reed–Solomon coding, the same family of codes used on CDs and in deep-space communication. When a scanner reads the code, it uses that extra data to detect and rebuild codewords that came out wrong because of a scratch, a smudge, glare, a fold or a logo sitting on top.

The four levels set how much of the code is spent on that redundancy. The percentages describe how much of the data can be restored, measured in codewords, not in area:

Because each codeword is made of 8 modules, a small mark that touches several codewords can use up more of the budget than its size suggests. Treat the percentages as an upper limit, not a target.

Redundancy takes space. For the same content, a higher level needs more codewords in total, so the generator moves up to a larger QR version with more modules. Here is the same 31-character link, encoded four times with the open-source encoder CirculateQR uses:

Printed at the same size, the level H version has smaller modules than the level L version, so it needs to be printed larger to scan from the same distance. That is the real cost of error correction. For the details on module size and distance, see the QR code size guide.

Those are theoretical maximums. A version 40 code is far too dense for a phone to read at any normal print size; real-world codes for phones are usually version 10 (57 × 57) or smaller.

A sensible default is M. Move up a level when the environment is rough or when you plan to cover part of the code. Move down only when you are short of space and the conditions are clean. If your content is long, consider a dynamic code first: a short link leaves plenty of room for a higher level without making the code dense.

A logo in the middle of a code hides modules, and error correction has to rebuild whatever they held. That is why logo codes should use level H. Some practical limits:

• Mind the area, not the width. A logo a quarter of the code’s width covers about 6% of its area (0.25 × 0.25). A logo a third of the width covers about 11%.
• Stay well inside the budget. Partly covered codewords count as damaged, so a logo uses more of the 30% budget than its area alone. Leave headroom for real-world wear, glare and printing errors.
• Never cover the finder patterns, the three large squares in the corners. Reed–Solomon coding does not protect them, and scanners need them to locate the code.
• Remove the modules under the logo and give it a clean background, rather than laying a semi-transparent image over the pattern, which confuses scanners.
• Test every version with several phones before printing. A logo that works at 5 cm may fail at 2 cm.

Error correction repairs damaged data. It does not rescue a code that a camera cannot find or resolve:

• Too small or too far away. If the modules blur together, every codeword is damaged.
• Low contrast. Pale colors, gloss glare or a busy background affect the whole code at once.
• A missing quiet zone. Without the blank margin the scanner may not find the code’s edges.
• Damaged finder patterns. Lose one of the three corner squares and many scanners give up.
• A wrong destination. A perfectly readable code pointing at a dead page is still a broken code. A dynamic code lets you fix the destination after printing.

Artistic codes bend the look of the modules, and error correction absorbs some of that. But a design that changes many modules at once can exhaust the budget quickly, and a higher level also means more, smaller modules for the art to work around. That is why a lower level with fewer, larger modules is sometimes easier to turn into a readable picture, as long as the result is checked with a real decoder.

In CirculateQR, plain and logo codes always use level H. For AI art, a density setting chooses the level: low (L), standard (M, the default) or high (H). Every AI image is then decoded automatically, and if the code cannot be read, the true pattern is redrawn into the artwork and checked again. The full process is in AI QR code art: how it works and how to make sure it scans, and the plans that include AI art are on the pricing page.