Convert Pixels per millimetre to Dots per pixel

Convert Pixels per millimetre (px/mm) to Dots per pixel (dppx) instantly and accurately.

Conversion Formula

dppx = px/mm × 0.2645833333

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.

About Dots per pixel

The dot per CSS-pixel unit (dppx, or the x descriptor in CSS) is defined in W3C CSS Images Level 3 / Media Queries Level 4 as the ratio of physical device pixels to CSS reference pixels, where the CSS spec fixes 1 CSS inch ≡ 96 CSS px exactly. Therefore 1 dppx = 96 DPI = 96 PPI exactly; a screen reporting device pixel ratio (DPR) of 2 has 2 dppx = 192 physical dots per CSS inch. JavaScript exposes DPR as window.devicePixelRatio, numerically equal to dppx. Reference values: standard laptop (96-110 PPI): 1 dppx; MacBook Pro 13-inch Retina (227 PPI): 2 dppx; iPhone 15 Pro 6.1-inch (460 PPI): 3 dppx; Samsung Galaxy S24 Ultra (501 PPI): ≈ 3.09 dppx. The HTML img srcset x descriptor (photo@2x.jpg 2x) is numerically equal to dppx, serving higher-resolution images for HiDPI screens. The CSS @media (min-resolution: 2dppx) query targets any display with DPR ≥ 2. 1 dppx = 96 DPI = 96 PPI.

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