QR Code Error Correction: L, M, Q and H Explained
Understand QR error-correction levels, why more redundancy can help damaged codes, and why higher is not automatically better for every design.
Quick answer
QR codes use Reed-Solomon error-correction codewords so a decoder can recover some missing or incorrect information. DENSO WAVE documents four selectable levels: L, M, Q, and H, with increasing recovery capability. Higher correction also consumes more symbol capacity, so it can make a QR denser for the same payload. Choose a level based on the operating environment, damage risk, logo treatment, size, and total data rather than automatically selecting the maximum.
Key points
- Error correction adds redundancy that helps recover corrupted codewords.
- The four commonly referenced levels are L, M, Q, and H.
- Higher correction increases resilience but uses more capacity.
- Physical damage pattern matters; a percentage is not a promise that any shape of damage will decode.
- Logo overlays increase the importance of careful testing and conservative design.
What to do
- 1Assess whether the code will face dirt, scratches, weather, labels, or logo obstruction.
- 2Choose an error-correction level that matches that environment.
- 3Regenerate the QR and review how the chosen level affects density.
- 4Keep critical position and structural patterns unobstructed.
- 5Test damaged or real-world samples if the deployment is important.
Common mistakes to avoid
- Assuming H makes every QR design safe regardless of how much is covered.
- Ignoring the density increase caused by higher redundancy.
- Treating approximate restoration percentages as guaranteed printable damage limits.
- Using error correction to compensate for poor contrast or undersized printing.
Frequently asked questions
What are QR error-correction levels L, M, Q and H?
They are selectable levels that add increasing amounts of error-correction redundancy to the symbol.
Which QR error-correction level should I use?
Choose based on damage risk, symbol density, size, environment, and any logo treatment. M is a common general-purpose starting point.
Is H always the best QR error-correction level?
No. H provides more redundancy but uses more capacity and can create a denser symbol.
Does error correction repair a damaged QR image?
The decoder can recover some corrupted data during scanning; it does not physically repair the image.
Can I cover part of a QR code with a logo?
Sometimes, but keep overlays conservative and test extensively. Error correction is not permission to cover arbitrary areas.
What is Reed-Solomon error correction?
It is the coding technique QR uses to add recoverable redundancy so some missing or incorrect codewords can be reconstructed.
Does higher error correction make the QR code bigger?
It can increase the required version or density for the same payload because more capacity is used for redundancy.
Can error correction fix poor contrast?
Not reliably. Detection and module sampling still depend on sufficient visual quality and contrast.
Can error correction make a QR waterproof?
No. It can improve tolerance to some damaged data, but physical materials and printing determine environmental durability.
Sources and further reading
Technical or policy-sensitive statements in this guide are grounded in primary or authoritative references where available.
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