Working with Dimensional Constraints in Revit

by John Flanagan

Lock a permanent dimension to create a constraint.

Use dimensional constraints to create relationships between elements in a building model.

When you place permanent linear or angular dimensions, a lock control appears with the dimension. Locks appear when you select:

• A permanent dimension.
• An element constrained by a dimension. An element is constrained if it is referenced by a locked segment or is a reference in an equality dimension.

If the lock is unlocked, the dimension value is modifiable and the element that it references is free to move in any direction. You can click the dimension value and change it.

If the lock is locked, the dimension value is fixed, preventing any change to the distance between its referenced components. You cannot click the dimension value to change it.

Fig 1:  Constraint from locked dimensions

Notice that the dimension value for the door is unlocked so the dimension value is modifiable and the element that it references can move in any direction. If you lock the dimension and move the grid line the door moves with it maintaining the 900 mm gap.

Fig 2:  Selected dimension with locked and unlocked controls

Many dimensions may correspond to the same constraint. For example, if you place a dimension between 2 walls and lock it, you have created a constraint. If you place another dimension between the same 2 walls and lock it, no additional constraint is created.

Tip: You can also switch the locked/unlocked state of the lock by right-clicking on it and choosing Toggle.