QR Codes Explained: How They Work and How to Avoid Common Mistakes

Updated 2026-08-03 · 6 min read

QR codes look like abstract noise, but almost everything in the pattern is doing a specific job. Understanding roughly how they work explains most of the reasons a code fails to scan.

What is actually in the pattern

The three large squares in the corners are finder patterns. They let a camera locate the code and work out its orientation, which is why a QR code scans upside down or at an angle.

A smaller square near the fourth corner is the alignment pattern, correcting for distortion when a code is photographed on a curved surface or from the side.

The rest holds your data plus a substantial block of error-correction information.

Error correction is the interesting part

QR codes deliberately store redundant data so they still work when partly damaged. There are four levels:

• L (Low) — recovers from roughly 7% damage
• M (Medium) — roughly 15%
• Q (Quartile) — roughly 25%
• H (High) — roughly 30%

This is what allows a code to work with a logo placed in the middle: the logo covers data, and error correction reconstructs it.

The trade-off is density. Higher correction means more redundant data, so the pattern becomes finer for the same content. A very dense code printed small can become harder to scan, which defeats the purpose. M is a sensible default; use H when adding a logo or printing somewhere the code may get scuffed.

Why codes fail to scan

Too small for the distance. A rough working rule is that the code should be about one tenth of the scanning distance. Read from 1 metre, aim for roughly 10 cm across. Codes on posters routinely fail because they were sized for a phone held close.

Not enough quiet zone. The blank margin around a code is functional, not decorative. Scanners use it to find the boundary. Cropping tight to the pattern is a common cause of failure.

Inverted colours. Most scanners expect dark modules on a light background. Light-on-dark works on some readers and fails on others.

Insufficient contrast. Mid-grey on light grey may look elegant and scan poorly. Keep contrast strong.

Too much data. A very long URL produces a dense pattern. Shortening the link first usually helps more than any other adjustment.

Static versus dynamic codes

A plain QR code is static: the destination is encoded in the pattern itself and can never be changed. Print it, and it points there forever.

Dynamic codes encode a short redirect URL controlled by a service, so the destination can be changed later. Useful, but with two real costs: it depends on that service continuing to exist, and it usually means someone tracking every scan.

For a printed run you may want to update later, dynamic makes sense. For a WiFi password on a café wall, static is simpler and depends on nobody.

Beyond links

QR codes can encode several formats phones recognise automatically:

• WiFi — network name, security type, and password, so guests connect without typing
• Phone or SMS — opens the dialer or a pre-filled message
• Email — pre-fills recipient, subject, and body
• Plain text — displays as-is

Worth remembering: a WiFi QR code contains your password in plain readable form. Anyone who photographs it has the password permanently. Fine for a guest network, unwise for your main one.

A word on safety

A QR code is unreadable to humans, which makes it an effective way to disguise a link. Scanning a sticker placed over a legitimate code on a parking meter or restaurant table is a known scam pattern.

The practical habit: check the URL your phone previews before opening it, and be sceptical of codes in public places that look stuck on rather than printed as part of the surface.