Rebar, or reinforcing bar, is a bar that's used to reinforce and strengthen concrete under tension. It's typically made of steel, though fiberglass is popular for certain projects because it doesn't corrode and is non-magnetic. There are many steel rebar sizes, grades, and types to support a variety of structures. The number represents a unit of 1/8 of an inch. So that means a #4 rebar is 4 times 1/8 inch or in other words - 1/2 inch. #5 bar is 5/8 inch; #6 bar is 3/4 inch and so on. Rebar size #3 is used for driveways and patios. For walls and columns, #4 rebar size should be used as they require more strength.