Here we go again - SOLIDWORKS is making it easy for you by using Global Variables and Equations.
Global Variables are independent variables that can be set to any numeric value.
Equations are used to create mathematical relationships between dimensions.
Let's go ahead and create some. There are two ways you can do this.
Here is the first way. If you know your design intent, setup those Global variables first.
We'll start by making a simple rectangular plate. "A" will be our width, "B" the length and "C" the thickness.
In a part file, click Tools, Equations.
Set your variables by clicking in the Add global variable cell, type the letter A and hit enter or the tab key. Type 50 after the equals (=) sign, hit enter/tab and that will prompt you to enter comments. Repeat the process for B=100 and C=5.
Click OK. An Equations folder becomes visible in the FeatureManager design tree. Expand the folder to see Global Variables.
Now let's assign our dimensions to the variables to create our equations. Draw a rectangle in a sketch and dimension a vertical line. In the modify box type an equal sign, hover over Global Variable and select A(50). Click the green check mark and now your 50mm is a Global Variable.
Click the green check mark again and repeat for the B value (one of the horizontal lines). You will notice both dimensions now have the epsilon symbol.
Let's extrude this 5mm and assign this value to our C Global Variable. For the depth, type equal sign and select C(5). Click green check. There you have it, now we can manage our design with ease. Simply right click Equations folder and enter new values. Click Automatically rebuild to have all changes take effect immediately. If this is not checked, you will need to rebuild yourself.
Here is another way we can do it. This way will create the Global Variable and the Equation all at once.
Create another Rectangular plate, working in the sketch, draw a rectangle and smart dimension a vertical line. In the modify box type an equal sign and A. Click the Global Variable symboland enter your value (in this case 50) and click the green check mark.
Right click equation folder and take a look. Global Variables and Equations are both created.
SOLIDWORKS wants you to focus on your design and not the software.