How this calculator works
The calculator subtracts the edge inset from both sides, then counts grid lines based on spacing in each direction. Total rebar length is the number of lengthwise lines times slab length plus the number of crosswise lines times slab width, with waste added. Stock bars are rounded up from total linear feet divided by bar length.
Example: a 20 ft by 12 ft slab with 18 in spacing and 3 in edge inset creates about 8 lines one way and 14 lines the other way. Total reinforcement rounds to 360.8 linear feet with 10% waste, or 19 pieces of 20 ft bar.