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A device-independent pixel is a logical unit used to size and position interface elements; the platform maps it to physical pixels on the display. That mapping helps keep an interface’s apparent dimensions more consistent across screens with different pixel densities. The unit is not universal: Android dp and Windows Direct2D DIP use different reference scales, while CSS pixels and iOS points are related abstractions with their own conventions.
Logical pixels and physical pixels
A physical pixel is a display’s smallest addressable picture element. A logical pixel is a measurement in a software coordinate system. The operating system or framework translates logical dimensions into physical pixels using its density or scaling rules.
That translation matters because the same number of physical pixels can occupy different apparent sizes on displays with different pixel densities. A layout expressed in a platform’s logical units is intended to avoid tying every dimension directly to a particular panel’s pixel count. It does not make screen sizes, aspect ratios, or usable layout areas identical; interfaces still need to adapt to the available space.
How Android dp and sp work
dp for interface dimensions
Android calls its density-independent layout unit a dp. Android’s design guidance uses a 160-dpi reference screen, where 1 dp is roughly 1 physical pixel, and gives this conversion: dp = (width in pixels × 160) / screen density. Android recommends specifying design dimensions in dp rather than assuming physical pixels have a consistent apparent size. See Android’s grids and units guidance and its screen-density guide.
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sp for text
Android’s scalable pixel, or sp, is intended for text that should respect a user’s preferred font size. Android says the default sp value equals dp, but the system adjusts rendered text size according to device settings. Use sp for text sizes when those preferences should be honored; dp remains the usual choice for layout dimensions. Details are in Android’s density documentation.
How Windows Direct2D DIP works
Microsoft defines a Direct2D device-independent pixel (DIP) as 1/96 of a logical inch. Its conversion is physical pixels = DIPs × DPI / 96. At 96 DPI, 1 DIP maps to 1 physical pixel; at 144 DPI, 1 DIP maps to 1.5 physical pixels. For example, 200 DIPs at 144 DPI render as 300 physical pixels. See Microsoft’s Direct2D and high-DPI documentation.
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Coordinate systems can differ even inside one application. Microsoft notes that Win32 mouse-event coordinates are physical pixels while Direct2D drawing uses DIPs. Convert coordinates when passing values between those systems; otherwise, input positions and drawn elements can fail to line up. The conversion issue is described in Microsoft’s high-DPI guidance.
How CSS pixels and device pixels relate
A CSS pixel is a web layout unit, not necessarily one physical pixel on the display. MDN defines a device pixel as a physical display unit and explains that devicePixelRatio reports the ratio of physical device pixels to CSS pixels for the current display. On a high-density display, a raster image or canvas may therefore use more physical pixels than its CSS dimensions. See MDN’s devicePixelRatio reference.
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How iOS points compare
iOS points are another logical coordinate unit. Apple’s archived view documentation describes coordinate values and distances as floating-point points and says a point does not necessarily correspond to one screen pixel; the system normally maps points in user coordinate space to pixels in device coordinate space. This is a useful conceptual comparison, not a claim that points equal Android dp or Windows DIP. Because the cited Apple page is archived, consult current Apple documentation for current API details. See Apple’s archived view documentation.
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How the units compare
| System | Logical unit | Baseline or relationship | Practical use |
|---|---|---|---|
| Android layout | dp | 1 dp is roughly 1 physical pixel at 160 dpi; dp = px × 160 / density. |
Use for layout dimensions. |
| Android text | sp | Default sp equals dp; rendered text also responds to user font-size settings. | Use for text that should honor those settings. |
| Windows Direct2D | DIP | 1 DIP = 1/96 logical inch; px = DIP × DPI / 96. |
Convert when physical-pixel coordinates enter DIP-based drawing. |
| Web | CSS px | devicePixelRatio reports physical device pixels per CSS pixel for the current display. |
Do not assume CSS dimensions are physical raster dimensions. |
| iOS (archived documentation) | Point | A floating-point logical coordinate mapped by the system to device pixels. | Comparable as an abstraction, not a guaranteed numeric match to another platform’s unit. |
What to use when building an interface
- Follow the target framework’s unit conventions for layout and drawing rather than treating physical pixels as a universal design unit.
- Use Android dp for layout dimensions and sp for text that should respond to user font-size preferences.
- When an API provides physical-pixel coordinates but another part of the application expects logical coordinates, convert between the coordinate systems explicitly.
- For web graphics, distinguish CSS layout dimensions from the physical pixel dimensions used to render a high-density display.
- Do not assume Android dp and Windows DIP represent the same physical size. Their reference scales and conversion formulas differ.
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