Rebar size and weight

Rebar is sized by a number that is also its diameter, once you know the trick: the bar number is eighths of an inch.

The table

#4 at 1/2 inch is the residential workhorse — slabs, footings, small walls. #3 appears in slabs on grade where the steel is there for crack control rather than strength.

Weight climbs with the square of diameter, so a #8 bar is not twice a #4, it is four times it. That is what makes a delivery estimate go wrong when a size gets substituted.

Spacing carries as much of the answer as size

Steel area per foot of slab is what a design actually calls for, and there are two ways to get it: bigger bars further apart, or smaller bars closer together. Closer and smaller distributes crack control better; bigger and wider is faster to tie.

bars needed = (span ÷ spacing) + 1

The +1 catches people. A 20 ft width at 12 in spacing is 21 bars, not 20 — both edges get one.

Cover is not optional and not decorative

Concrete protects steel from moisture. Too little cover and the bar corrodes, expands, and spalls the slab off from the inside; too much and the steel sits outside the tension zone where it does nothing.

Rebar dumped on the sub-grade and "pulled up during the pour" ends up on the bottom, which is the same as having no steel in a slab loaded from above. Chairs cost almost nothing.