Choose Tools tab→Variables group→Variables 
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In the Variable Table, in the Unit Type list, select the unit type for the variable. The default type is Distance, which means that any expression created will be for a distance. For example, if you want to create a variable to control an angular dimension, you must select Angular.
Note:
To store a value without units, select Scalar from the list.
In the Name column, click an empty cell.
Type a name for the variable that you want to create. Press Enter.
In the same row, in the Formula column, do one of the following:
Type the value that you want for the variable.
Type a simple expression for the variable. A simple expression consists of using standard mathematical operators to set up an equation using values and other variables. For example, if variables Var1 and Var2 already exist, you can type the following formulas when creating a new variable:
Var1 * Var2
Var1 / 3.0
(Var2 + 5.0) * Var1
Var2 ^ 2
Click the Check button to create the variable.