A driven dimension is a dimension in CAD software that reports a measurement rather than controlling it. Unlike a standard (driving) dimension where you type in a value and the geometry obeys, a driven dimension reads the current state of your sketch or assembly and displays whatever value the geometry happens to be. You can’t edit a driven dimension to reshape your model, because its value is determined by other dimensions, constraints, or component positions that already define that geometry.
Driving vs. Driven Dimensions
Every dimension in a CAD sketch or assembly falls into one of two roles. A driving dimension acts like a command: set it to 50 mm, and the geometry moves to 50 mm. A driven dimension acts like a readout: it measures whatever is already there and updates automatically when something else changes the geometry.
The distinction matters because CAD models are built on precise mathematical relationships. Each piece of geometry has a certain number of degrees of freedom, meaning the directions it can still move. A driving dimension locks down one of those degrees of freedom. A driven dimension doesn’t lock anything. It simply watches and reports.
In most CAD programs, driven dimensions are visually distinguished from driving ones. SolidWorks places them in parentheses, for example, so you can tell at a glance which dimensions are controlling geometry and which are just measuring it.
Why Driven Dimensions Exist
The most common reason you’ll encounter a driven dimension is to avoid over-constraining a sketch. Every sketch has a specific number of degrees of freedom, and once they’re all locked down by dimensions and constraints, the sketch is “fully defined.” If you try to add one more driving dimension at that point, the software has a problem: two competing instructions for the same piece of geometry.
In Autodesk Fusion, this triggers a warning that reads: “Adding the dimension will overconstrain the sketch. Choose OK to create a Driven Dimension.” The software is telling you that the geometry is already controlled, so this new dimension can only exist as a reference measurement, not as a command. Most other parametric CAD tools handle the situation similarly.
This is genuinely useful. Say you’ve drawn a triangle and fully defined it with three side lengths. You might still want to know one of the interior angles. Adding that angle as a driving dimension would create a conflict, but adding it as a driven dimension gives you the information without breaking anything.
Practical Uses in Design
Driven dimensions aren’t just a fallback for over-constrained sketches. They serve real design purposes.
In assemblies, you can set a distance or angle between components as a driven dimension so it reflects how parts move relative to each other. If you’re modeling a hinge mechanism, for instance, the opening angle might be controlled by one component’s rotation. A driven dimension on that angle lets you read its value at any position without forcing it to stay fixed.
Driven dimensions are also valuable for inspection and documentation. Engineers often need to call out a measurement on a drawing that isn’t directly defined in the model but is a consequence of other geometry. An overall length that results from several smaller features, a diagonal distance across a part, or the gap between two components in a specific configuration can all be captured as driven dimensions that stay accurate as the design changes.
Some CAD platforms take the concept further with dimension-driven design, where relationships between dimensions are defined by formulas. In Bentley’s MicroStation, for example, you can create a rectangle where the width is always half the length. Define the length, and the width is calculated from the constraint formula. When you modify the length later, the width updates automatically to maintain that ratio. The width in this case behaves as a driven value, determined entirely by the length and the formula connecting them.
How to Convert Between Driving and Driven
Most parametric CAD tools let you toggle a dimension between driving and driven. In SolidWorks, you right-click a dimension and select “Driven” from the context menu. In Fusion 360, the software prompts you during placement if the dimension would over-constrain the sketch, and you can accept the driven option directly.
Converting in the other direction is also possible but requires more thought. To turn a driven dimension into a driving one, you typically need to remove or change another dimension or constraint first to free up the degree of freedom. Otherwise you’re back to the over-constrained conflict the software was trying to prevent.
A useful workflow is to sketch geometry loosely, add the dimensions you care about most as driving dimensions, and then add any secondary measurements you want to track as driven dimensions. This keeps your design intent clear: the driving dimensions define what matters, and the driven dimensions let you monitor everything else.
Common Mistakes to Avoid
The biggest mistake beginners make is treating a driven dimension like it’s broken. When you double-click a driven dimension and find you can’t change the geometry, that’s by design. If you need to control that value, you have to rethink which dimensions are driving and which are driven.
Another common issue is cluttering a sketch with too many driven dimensions, making it hard to tell which dimensions actually control the geometry. A clean sketch has a clear set of driving dimensions that match your design intent, with driven dimensions used sparingly for reference values you genuinely need to monitor.
Finally, be careful when importing sketches or working with templates created by someone else. If key dimensions are set as driven when you expected them to be driving, you may find yourself unable to edit the geometry the way you want. Check the dimension type before assuming a sketch is misbehaving.