Minimum size of a QR code: how big it needs to be to scan reliably on any medium

The general rule for QR size is that the code must measure at least one tenth of the scanning distance. If the user scans at 30 cm, the QR must be at least 3 cm x 3 cm. If scanning at 1 metre, at least 10 cm x 10 cm.

This proportion is called the '1:10 rule' and is the standard recommendation of the ISO/IEC 18004 specification, the international QR code standard.

In practice, the cameras of modern smartphones (from 2019 onwards) have autofocus and image processing sufficient to read QR codes below that proportion, but there is no guarantee it will work on older devices or those with lower-quality cameras. For printed materials that will last years, using the 1:10 proportion is the safe decision.

Contrast also matters: a black QR on a pure white background reads at a smaller size than one with low-contrast custom colours. If you customise the QR colours, always check that the contrast between the modules and the background is at least 3:1.

This table summarises the recommended sizes by medium and typical reading distance:

| Medium | Typical scanning distance | Recommended minimum size | |--------|--------------------------|---------------------------| | Business card | 15-20 cm | 2 cm x 2 cm | | Product label | 20-30 cm | 2.5 cm x 2.5 cm | | A5 flyer or leaflet | 30-40 cm | 3 cm x 3 cm | | A4 brochure or catalogue | 30-40 cm | 3 cm x 3 cm | | A3 indoor poster | 50-80 cm | 5 cm x 5 cm | | A1 poster | 80-120 cm | 8 cm x 8 cm | | Outdoor street poster | 100-200 cm | 10-20 cm x 10-20 cm | | Advertising billboard | 3-10 metres | 30-100 cm x 30-100 cm |

These sizes are for QR codes with error correction level M or H. With level L, the QR can be slightly smaller because it has fewer correction modules, but it is also more fragile in the face of damage or dirt.

The QR standard defines four error correction levels:

| Level | Recoverable damage | Ideal for | |-------|-------------------|-----------| | L (Low) | 7% of the code | Clean environments, inner packaging | | M (Medium) | 15% of the code | General use, flyers, posters | | Q (Quartile) | 25% of the code | Environments with some dirt | | H (High) | 30% of the code | QR with central logo, outdoor |

The higher the correction level, the more redundancy modules the QR includes. This means that for the same content, a QR at level H has more dots than one at level L, and therefore needs to be larger for the modules to be readable by the camera.

Practical rule:
• Without a logo: use level M. Minimum size as per the table above.
• With a logo (covering part of the QR): use level H without exception. Add 15-20% to the minimum size to compensate for the higher module density.

In the codigo-qr.es generator you can choose the error correction level when creating the QR. The Pro plan allows logo addition with level H automatically.

The surface affects QR readability in two ways: the actual contrast (which can differ from the digital file's contrast) and geometric distortion.

Surfaces that allow smaller sizes:
• White matte paper: maximum contrast, no reflections. Allows reaching the minimums in the table above.
• White cardboard: similar to matte paper, slightly thicker but equally high contrast.

Surfaces that require larger sizes:
• Glossy paper or material: light reflections reduce effective contrast. Add 20% to the minimum size.
• Plastic or vinyl: same reflection problem. Add 20-30%.
• Fabric or rough surfaces: texture distorts the edges of the modules. Add 30-50% and use correction level H.
• Curved surfaces (cups, bottles): the curvature deforms the QR when scanning. Always test scanning before production and increase the size until it is reliable.
• Anodised metal or wood: lower contrast and texture. Requires prior testing and generous sizing.

Always print a real-scale sample and scan it with three different devices (including a budget one) before approving mass production.