Why won't my QR code scan?
Almost every failed scan comes down to one of eight causes. Here is how to tell which.
The eight causes
| Cause | Symptom | Fix |
|---|---|---|
| Too small | Camera never locks on | Print at 2cm minimum, 3cm+ for long URLs |
| Low contrast | Partially detected, then lost | Dark code on light background, not grey on white |
| Glare | Works at an angle only | Tilt the surface, or move the light source |
| Crease or damage | Fails only across the crease | Flatten, or reprint |
| Moiré on screens | Interference bands in the camera | Screenshot and decode the image instead |
| Blur or motion | Focus hunting | Tap to focus, hold still, brace the phone |
| Inverted colours | Never detected | Use dark-on-light; some decoders handle inverted, many do not |
| Cropped finder pattern | No detection at all | All three corner squares must be intact |
The three corner squares matter most
A QR code has three large square markers in three corners. They are how the decoder finds the code and determines its orientation. If a design crops one off, cuts a logo over one, or wraps the code around a curved surface so one marker is distorted, no decoder will read it — error correction cannot help, because the code cannot be located in the first place.
Designers add logos over the centre of QR codes, which is usually fine because the centre is data, protected by error correction. Covering a corner marker is not fine, and it is a common mistake in printed materials.
If it exists on a screen, do not photograph it
Screenshot and decode the image. Photographing a monitor introduces moiré, perspective skew and uneven brightness, all of which degrade exactly the edges a decoder needs. The original pixels are strictly better.
This is also the fastest fix when a phone camera refuses to focus: screenshot, then use an image-based reader.
Frequently asked questions
How small can a printed QR code be?
About 2cm for a short URL at close range. Longer content needs more modules, so the physical size must grow to keep each module printable — 3-4cm for a typical link.
Can a damaged code still work?
Yes, up to the error correction level — typically 7% to 30% of the code can be lost. Damage to a corner marker is the exception: it usually cannot be recovered.
Why does the same code work on one phone and not another?
Camera quality, autofocus behaviour and the decoder version. Older or cheaper cameras struggle with small, glossy or low-contrast codes.