Flutter Layout Explained: How Constraints, Sizes, and Widget Trees Work

Flutter's layout system follows a single rule at every level of the widget tree: constraints pass down from parent to child, sizes return upward, and the parent sets the child's position. Each widget operates within a three-tree architecture — the widget tree, element tree, and RenderObject tree — where actual layout calculations happen in the RenderObject layer. Common single-child widgets like Container, Padding, Align, SizedBox, and ConstrainedBox each play a distinct role in controlling size, spacing, and positioning. Constraints can be tight, loose, or unbounded, and unbounded constraints inside unconstrained parent widgets are a frequent source of layout errors in Flutter. Understanding this top-down, single-pass system explains both Flutter's rendering speed and why certain layout patterns require specialised widgets to implement correctly.