You can use formulas to modify the dimensions of specific elements.
For example, as you sketch a wall, you can enter a formula for the temporary dimension of the wall. Begin the formula with an equal sign, and then use conventional mathematical syntax.
To modify a dimension for an existing element
Revit displays a temporary dimension. (If the dimension uses the wrong reference points, drag the blue squares (of the witness lines) to the desired reference points.)
You may need to zoom in to see the dimension clearly. Revit displays its value in a text box.
Valid formula for setting wall length