How the rebar calculator works
One bar per spacing interval, plus one at the edge, in both directions. Across a 20 ft slab at 12 inch spacing that is 21 bars each way (20 x 12 / 12 + 1), for 840 linear feet of rebar in total. Weight per foot depends on the bar size: #3 is 0.376 lb/ft, #4 is 0.668 lb/ft, #5 is 1.043 lb/ft and #6 is 1.502 lb/ft.
So 840 linear feet weighs 561.12 lb in #4 bar, or 315.84 lb in #3 bar. Lap length where bars join (usually 30–40 bar diameters) is not included — add it for long runs, and use chairs to hold the grid off the ground before the pour.
Working out the concrete for the slab itself? Use the concrete calculator.