There’s more than one way to skin a cat, as the old saying goes. There’s also more than one way to create rounded corner rectangles in Adobe Illustrator. The Rounded Rectangle Tool in Illustrator is handy, but it’s limited in that once you create the rectangle, you can’t go back and alter the amount of the rounded corners later on if you need to. Fortunately, Illustrator offers another, more flexible method to accomplish the task. 
To get around this limitation, create a normal squared-edge rectangle. With the rectangle selected, go to Effect>Stylize>Round Corners… in the menubar. 
The Round Corners dialog box allows you to set a custom Radius to your rectangle, and tick the Preview box so you can see what your rectangle will look like. But here’s where the handy part comes into play. When you create the round corners this way, you can go back and adjust the radius amount at any time later on in your design process because the round corners are a live effect, just like fill, stroke, opacity, etc. To do that, select the rectangle and simply visit the Appearance Panel. You’ll see the Round Corners effect listed in the panel along with any other attributes applied to the rectangle. Double-click the effect and you can edit the radius of your rounded corners.
Unfortunately, the round corner effect is not accurate. If the angle of the corner is not 90 degrees, the radius will not be the radius you enter. The sharper the corner, the smaller the radius, and on >90 degree angles the radius becomes larger than what you enter. Also is the rounding effect not always following a perfect circle.
If you are serious on rounded corners, draw a shape in AutoCAD and then choose: fillet, radius, diameter, polyline and click the shape to give rounded corners. Import the .dwg in Illustrator for perfect corners, not the arbitrary crap Illustrator produces.