Density independent pixel

A density independent pixel is a unit of length that does not care how finely a screen is made. Android anchors it to a reference display of 160 dots per inch, so one dp is one physical pixel there and the conversion everywhere else is the ratio: at 3x, a 48 dp button is painted with 144 device pixels and comes out exactly as large under the thumb. Pixel density is how many device pixels a screen packs into an inch; a dp is the unit that cancels it out, so a control stays thumb sized on every phone that ships. The pairing is the whole idea. One number describes the layout, and the platform does the multiplication.

Every platform has this unit and each gave it a different name, which is most of the confusion around it. Android says dp. Apple says point, which is not the typographic point despite sharing the word, and multiplies it by the device’s scale factor. Microsoft says effective pixel. The web says CSS pixel, anchored to roughly 96 per inch rather than 160, and exposes the multiplier as the device pixel ratio. The arithmetic differs, the idea does not. Android adds one more unit, sp, which is a dp that also scales with the reader’s chosen text size, and it is the correct unit for anything made of type.

The unit is a reference, not a physical measure, and that is worth being precise about. It is calibrated to a typical viewing distance as much as to an inch, which is why a tablet held at arm’s length and a phone held close can share a number and still feel right, and why a dp on a television is not a dp on a watch. What it reliably buys is that a viewport is measured in the same currency the layout is written in, so density never moves a breakpoint and a touch target specified at 48 dp is 48 dp everywhere.

In practice the rule is that device pixels appear in exactly two conversations. The first is raster assets, which need enough samples for the densest screen they will land on. The second is hairlines, where a one dp rule is drawn with two or three device pixels and can look heavier than intended, which is why some systems specify their dividers in device pixels on purpose. Everything else, spacing, radii, target sizes, type, belongs in the density independent unit, and a physical pixel value anywhere in a layout file is almost always a bug waiting for a denser phone.