Dimensions & constraints
Lock a sketch down with dimensions and geometric relations so edits stay predictable.
A sketch you can drag around freely is a sketch that will surprise you later. Dimensions and relations pin its shape so a change does exactly what you intend.
Smart Dimension
- Start Smart Dimension, then click what to measure: a line → length · two lines → angle or distance · a circle → diameter · an arc → radius.
- Place the label, type the value,
Enter. - Double-click any dimension later to change it — the solver updates the geometry live.
Geometric relations
Select one or two entities and apply:
- Parallel · Perpendicular
- Tangent — line↔arc, arc↔arc
- Horizontal / Vertical — for lines or between two points
- Coincident — glue a point onto another point or curve
- Equal — same length / radius
- Distance and Angle — driven values between entities
Relations solve live: drag any point and the rest follows the rules.
Under-defined vs fully defined
While parts of a sketch can still move, they are under-defined. Keep adding dimensions and relations until nothing drags — a fully defined sketch never shifts when something upstream changes. The AI assistant can finish the job for you: ask it to "fully define this sketch" (Using the AI assistant).
When it fails
- Over-defined / conflicting — the newest relation contradicts an earlier one; delete one of them. Dimensions on top of an Equal relation are the usual culprit.
- Everything jumped after an edit — the sketch was under-defined; the solver picked the nearest valid answer. Add relations first, then re-try the edit.
Related: Sketching basics · Fix a rebuild error