QR Codes on Thermal Label Printers: 203 DPI and 300 DPI Module Math

1. Dots, Not Millimeters

A 203 dpi printhead lays down about 8 dots per mm, so one dot is about 0.125 mm. A 300 dpi head has dots about 0.085 mm.

2. Use Whole-Dot Modules

Fractional scaling creates uneven modules. Pick an integer number of dots per module: * 203 dpi, 4 dots per module = 0.5 mm * 300 dpi, 6 dots per module = about 0.51 mm

3. Worked Example

A Version 2 code (25 modules) plus an 8-module quiet zone is 33 modules. At 4 dots each on 203 dpi: $$33 \times 4 = 132\text{ dots} \approx 16.5\text{ mm}$$

4. Tips

Set darkness and speed so modules neither fade nor bloom, and print the code from a vector or exact-pixel bitmap, not a resampled image.