Most AutoCAD users draw with object snaps and careful editing. That works until a client asks for the bracket to be 20 mm wider and you have to stretch, re-trim and re-check every connected line. Parametric constraints solve that problem. They let you tell AutoCAD how objects relate to each other (this line stays horizontal, these two circles share a center) and how big things must be (this edge is 120 long). Change one value and the geometry updates while keeping the rules you set. This tutorial walks through both kinds of constraints, geometric and dimensional, with practical steps you can try today.
Geometric vs. Dimensional Constraints: The Basics
Geometric constraints control relationships between 2D objects or points. According to Autodesk's help documentation, they include rules such as perpendicular, parallel, tangent, concentric, coincident, horizontal, vertical, symmetric, equal, collinear, smooth and fix. A coincident constraint keeps two endpoints glued together; a fix constraint locks a point or object to a location relative to the World Coordinate System so that it acts as an anchor while everything else adjusts around it.
Dimensional constraints control sizes: distances, lengths, angles, radii and diameters. Each one creates a named variable such as d1 or dia1. You can rename those variables and even drive one from another with formulas, which is where the real power of parametric drawing appears.
One important note: AutoCAD LT has more limited parametric tools than full AutoCAD, so some commands described below may not be available in every product.
How to Apply Geometric Constraints
You will find the tools on the Parametric tab of the ribbon. The workflow is simple, but the order in which you pick objects matters, because by default the second object moves to match the first.
- Draw a rough shape, for example a closed polyline or a few lines and an arc. Do not worry about exact sizes yet.
- On the Parametric tab, choose a geometric constraint such as Horizontal, then select the bottom edge.
- Apply Perpendicular between the bottom edge and a side edge. Pick the edge you want to stay put first.
- Add a Fix constraint to one corner so the shape has an anchor point.
- Use Equal or Parallel where the design requires it, for example to keep two holes the same size.
- Hover over objects to check the constraint icons. Coincident constraints show as small blue squares, while other constraints appear as gray icons.
If applying constraints one by one feels slow, try AUTOCONSTRAIN. It applies essential constraints to a selection set automatically, based on settings that control which constraints are used, their priority and the tolerances for detecting horizontal, vertical or touching objects. Expect to add or remove a few constraints manually afterwards.
You can also turn on Infer Constraints. With it enabled, commands such as LINE, PLINE, RECTANG, FILLET and CHAMFER create some constraints for you as you draw. For instance, RECTANG adds parallel and perpendicular constraints, and FILLET adds tangent and coincident constraints between the new arc and the adjacent lines. Fix, symmetric, equal and a few others are never inferred.
How to Apply Dimensional Constraints
Once the shape behaves correctly, lock in the sizes. The general command is DIMCONSTRAINT, and there are dedicated commands for each type: DCLINEAR, DCHORIZONTAL, DCVERTICAL, DCALIGNED, DCANGULAR, DCRADIUS and DCDIAMETER.
- Start DCHORIZONTAL and pick two points along the bottom edge (endpoints and midpoints are valid points on a line).
- Place the constraint and type a value, or an expression such as width=120.
- Add a vertical constraint for the height, and a DCRADIUS or DCDIAMETER constraint for any holes.
- Double-click any constraint value later to edit it and watch the geometry update.
- Already have associative dimensions? Use DCCONVERT to turn them into dimensional constraints.
Dimensional constraints come in two forms. Dynamic constraints are the default: they keep the same size on screen when you zoom, use a fixed style, can be hidden globally and do not plot. Annotational constraints use the current dimension style, scale with the drawing and do plot. You can switch between them in the Properties palette or with DCFORM. There are also reference parameters, which show a measurement in parentheses without driving the geometry.
Drive Your Design with the Parameters Manager
The Parameters Manager lists every dimensional constraint, reference parameter and user variable in the drawing. Open it with the PARAMETERS command or from the Parametric tab. Rename d1 to something meaningful like width, edit expressions, and create user parameters not tied to any object.
Expressions support the usual operators (+, -, *, /, ^ and parentheses) plus functions such as sqrt, round, min, max, sin and cos, and constants like pi. That means you can set height = width / 2, or make a hole diameter equal to a plate thickness multiplied by a factor. Change a single user parameter and the whole profile updates. If you work in imperial units, put a space around the minus sign so AutoCAD does not read it as a feet-inch separator.
Constraints pair nicely with blocks too. If you build reusable parts, see our guide on how to make a dynamic block in AutoCAD, where the same parametric ideas can be used inside the Block Editor.
Tips to Avoid Constraint Headaches
- Fix first. Anchor one point early so the drawing does not drift when values change.
- Constrain shape before size. Apply geometric constraints first, then dimensional ones.
- Constraints cannot be edited. To change a geometric constraint, delete it and apply a new one. DELCONSTRAINT removes all constraints from a selection.
- Use meaningful names. width and hole_dia are far easier to manage than d7 and dia3.
Conclusion
Parametric constraints turn a static AutoCAD drawing into a small, rule-based model. Geometric constraints keep the shape logical, dimensional constraints control the sizes, and the Parameters Manager lets you drive everything from a few named values. Start with a simple plate or bracket, practice the steps above, and you will quickly see how much redrawing it saves on design changes.