0dp

0dp is a dimension value used in Android layout XML. The unit dp, short for density-independent pixels, allows measurements to scale with screen density. A value expressed in dp is converted to physical pixels at runtime. A value of 0dp thus represents zero density-independent pixels, which equals zero pixels on any screen density.

In ConstraintLayout, 0dp has a special meaning. It indicates that the dimension should be determined by constraints rather than a fixed size, a mode commonly described as match_constraints. For example, setting a view’s width to 0dp and constraining its left and right edges to the parent makes the view stretch to fill the available horizontal space. Similarly, setting height to 0dp with top and bottom constraints makes the view fill vertical space. 0dp is often used to create flexible layouts, such as creating a view that adapts to other elements in a chain or is governed by layout constraints and bias.

Outside ConstraintLayout, 0dp simply means a size of zero pixels and is rarely useful for visible elements. It does not imply a fixed or fill behavior in non-constraint contexts. Understanding 0dp is therefore twofold: it is a zero-size value in the general Android unit system, and it is a dynamic sizing signal within ConstraintLayout to effectively match constraints.