Convert Dots per inch to Pixels per millimetre

Convert Dots per inch (DPI) to Pixels per millimetre (px/mm) instantly and accurately.

Conversion Formula

px/mm = DPI × 0.03937007874

About Dots per inch

The dot per inch (DPI) is the standard resolution metric for printed output, specifying how many individual ink or toner dots a printing device places per linear inch. Unlike a pixel, a printed dot is a physical deposit of ink variable in size and subject to dot gain (ink spread into substrate). Inkjet photo printers: Epson SureColor SC-P900 up to 5760 DPI; HP DesignJet Z9+Pro at 2400 DPI. Laser printers: HP LaserJet standard 600 DPI; enterprise class 1200 DPI. Offset CTP plate imaging: 2400-3600 DPI. Rule of thumb: required DPI ≥ 1.5 × LPI for acceptable halftone; ≥ 2 × LPI for high quality - a 175 lpi magazine job needs ≥ 350 DPI raster data. Thermal dye-sublimation printers achieve continuous-tone output at 300 DPI because dye diffuses to fill each cell. As a dimensional unit, 1 DPI = 1 PPI; DPI implies physical print output. 1 DPI = 2.54 dots/cm = 25.4 dots/mm.

About Pixels per millimetre

The pixel per millimetre (px/mm) is the resolution unit of precision in machine vision, flatbed scanning, and microscopy, where working distances are in millimetres. Industrial camera datasheets (Basler acA5472-17um, FLIR BFS-U3-200S6M-C) specify resolution in px/mm at defined field widths; a 20 MP sensor over a 100 mm field resolves 54.96 px/mm. ISO 12654 (archival scanning) specifies minimum 8 px/mm (≈ 200 PPI) for standard digitisation and 24 px/mm (≈ 600 PPI) for fine-detail originals. Digital pathology: Leica Aperio GT450 at 40× magnification achieves ≈ 4,405 px/mm. CMOS sensor pixel pitch has shrunk from 7.4 µm/px (≈ 135 px/mm, mid-2000s) to 0.64 µm/px (≈ 15,625 px/mm) for smartphone sensors in 2024. Since 1 in = 25.4 mm exactly, 1 px/mm = 25.4 PPI exactly. A 300 PPI photo print = 300/25.4 ≈ 11.81 px/mm.

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