Scalle evenly... is it possible?

I have a problem. I am trying to scale evenly a rectangle, but the scale is done proportional not in fixed amount and as a result there is a difference between rectangles. Is there a way to make the a=b ?

Ensure you have applied any scale (Ctrl+A / scale in object mode) to ensure the scale of each axis is 1
Then use the inset tool (I). Press F6 or look in toolshelf (T) for inset options

Apply Scale (Ctrl+A) and Inset Faces (I).

Thank you, it is exactly what I need.