AUTOCONSTRAIN command in AutoCAD : Applies geometric constraints to a selection set of objects based on orientation of the objects relative to one another

If you need to apply geometric constraints quickly to a set of objects in AutoCAD, the AUTOCONSTRAIN command can save a lot of time. This guide explains what Auto Constrain does, how to use it step‑by‑step, why it might fail, useful alternatives, troubleshooting steps, and practical tips to get predictable results.

What is the AUTOCONSTRAIN command?

AUTOCONSTRAIN (also shown in the ribbon as Auto Constrain) is an AutoCAD tool that automatically applies geometric constraints to a selected set of objects based on their orientation and relationships. It attempts to Infer constraints such as:

• Coincident
• Collinear
• Concentric
• Parallel
• Perpendicular
• Tangent
• Horizontal / Vertical
• and other common geometric constraints

The command is part of AutoCAD’s Parametric toolset and helps convert a loose drawing into a parametrically constrained one so relationships are maintained when you edit geometry.

When and why to use Auto Constrain

Use Auto Constrain when you want to:

• Quickly capture the geometric relationships in a 2D sketch or layout.
• Prepare a drawing for parametric edits, so moving one object updates related objects.
• Convert a manual drawing into a constrained model for later dimension-driven design.
• Speed up work when many obvious constraints exist and you don’t want to add them one‑by‑one.

Example result: a set of line segments that were only visually aligned becomes constrained so lines stay parallel or perpendicular after edits.

How to use AUTOCONSTRAIN — Step by step

Prepare your drawing

1. Make sure the geometry you want to constrain is in Model space and is primarily 2D (Auto Constrain works best on 2D geometry).
2. Explode or edit blocks/XREFs (see troubleshooting section) if you intend to constrain objects inside them.
3. Turn on object snaps if you need to clean up geometry beforehand (use ENDPOINT, MIDPOINT, etc.).

Using the Ribbon

1. Switch to the Parametric tab on the ribbon.
2. In the Geometric panel, click Auto Constrain.
3. Select the objects you want to constrain, then press Enter.
4. Auto Constrain will apply inferred geometric constraints and display the constraint symbols.

Using the Command Line

1. Type AUTOCONSTRAIN and press Enter.
2. Select objects and press Enter.
3. Auto Constrain runs and applies constraints automatically.

Useful selection tips

• Select a focused set of objects rather than the entire drawing to avoid unwanted constraints.
• Use selection windows to include only the geometry you want constrained.
• If results look wrong, undo (Ctrl+Z), refine your selection or clean up geometry, and try again.

Example workflow (before → after)

• Before: a rectangle drawn with four separate lines, visually aligned but not linked.
• Run AUTOCONSTRAIN on the four lines → After: opposite sides become parallel, corners become coincident, and the rectangle behaves as a single constrained unit when moved or adjusted.

Common problems and fixes (Why Auto Constrain doesn’t work)

If Auto Constrain seems to fail or applies no constraints, check these common causes and fixes:

• Objects are part of a Block or XREF

  • Problem: constraints cannot be added to geometry inside a locked or External reference.
  • Fix: Explode the block (or use REFEDIT to edit in place), or constrain geometry before inserting it as a block.

• Geometry is not truly coincident or contains small gaps

  • Problem: endpoints that look joined may be separated by a tiny distance.
  • Fix: use JOIN, TRIM/EXTEND, or snap endpoints together to remove gaps.

• Objects are 3D or not planar

  • Problem: Auto Constrain expects 2D planar geometry.
  • Fix: flatten 3D objects or work on a 2D projection before constraining.

• Existing constraints conflict

  • Problem: pre-existing constraints may prevent adding new ones.
  • Fix: examine and remove conflicting constraints (select constraint symbols and delete or use the Delete Constraints tool), then re-run Auto Constrain.

• Constraint visibility is off

  • Problem: constraints are present but not visible.
  • Fix: turn on constraint display. In many AutoCAD versions you can toggle constraint visibility from the Parametric tab or by setting the system variable CONSTRAINTDISPLAY (set to 1 to show).

• Layer or object is locked

  • Problem: locked layers or objects cannot be modified.
  • Fix: unlock the layer or objects before running Auto Constrain.

• Selection included unsupported entities (like raster images or text)

  • Problem: Auto Constrain ignores unsupported entity types.
  • Fix: select only geometric entities (lines, arcs, circles, polylines).

If Auto Constrain still gives unexpected results, try constraining a very small, simple set (two to four objects) to test behavior, then expand gradually.

Alternative methods and manual constraint tools

If Auto Constrain is not appropriate or you want finer control, use manual constraint workflows:

• Use the Parametric > Geometric toolbar or ribbon to apply constraints one-by-one: Coincident, Collinear, Concentric, Parallel, Perpendicular, Tangent, Horizontal, Vertical, Equal, etc.
• Use dimensional constraints (in the Parametric tab) to add parameter‑driven distances and angles that control geometry.
• Convert a careful measured drawing into a parametric sketch by manually placing a few key constraints and dimensions rather than auto applying many inferred constraints.
• Use dynamic blocks to encapsulate common parametric behaviors if you need re‑usable constrained components.

Manual constraint application is slower but gives predictable, high‑quality constraint structures and avoids irrelevant or conflicting constraints that Auto Constrain might produce.

Best practices and tips

• Work in 2D and on a clean layer structure; keep constraints on a dedicated constraint layer if you want to control visibility and editing.
• Always save or use a backup before applying Auto Constrain to large selections — you can undo if results are unwanted.
• Apply Auto Constrain incrementally: constrain small areas first, verify results, then expand selection.
• Use object snaps and geometric cleanup (join, trim, extend) before constraining to reduce errors.
• If you will share the file, keep constraint symbols visible or document constraints so collaborators understand the parametric relationships.
• To remove constraints in bulk, select and delete the constraint objects or use ribbon tools on the Parametric tab that manage constraints.

FAQ

Can I undo Auto Constrain if the results are wrong?

Yes — you can use Undo (Ctrl+Z) immediately after applying Auto Constrain. If you’ve saved and closed the file, you can remove constraints manually by selecting and deleting the constraint symbols or using the Delete Constraints tool on the Parametric tab.

Will Auto Constrain change my geometry shape or sizes?

Auto Constrain only adds constraints (relationships) and does not directly change geometry coordinates. However, if constraints add degrees of restriction that conflict with existing dimensions, subsequent edits may appear to move geometry. Always review constraints after applying them.

Does Auto Constrain work on 3D objects?

Auto Constrain is intended for 2D planar geometry. It may not produce meaningful results on 3D solids or non‑planar geometry. For 3D, consider other modeling constraints or use 2D sketches projected onto a plane.

How can I remove all constraints from my drawing?

You can remove constraints by selecting constraint icons and deleting them, or by using the constraint management tools in the Parametric tab (such as Delete Constraints). If constraints are on a dedicated layer, you can also turn that layer off or delete it after ensuring you no longer need the constraints.

Why are some constraints not applied even though geometry looks aligned?

Tiny gaps, nearly parallel lines that are slightly angled, or objects existing as separate entities (blocks/XREFs) are common causes. Use tools like JOIN, snap endpoints, explode blocks, or clean geometry first, then re-run Auto Constrain.

Can I customize which constraint types Auto Constrain will apply?

Auto Constrain uses internal logic to infer constraints. For precise control, prefer manual constraint tools from the Parametric tab to choose exactly which constraints to add. Some versions of AutoCAD may offer settings or tolerance options that affect inference behavior — check your version’s Parametric settings.