How a rebar grid is measured
Rebar runs in two perpendicular directions over the slab: bars across the width at your spacing, and bars along the length between them. The calculator counts both sets — rows across plus columns along — converts each to its full bar length, and sums the linear footage. <strong>18-inch spacing</strong> is the common residential choice; 12-inch spacing doubles the steel for heavy loads.
Placement beats quantity
Rebar belongs in the middle third of the slab — on chairs so the pour pushes it down to center, not lying on the ground. Overlap splices 40 times the bar diameter (about 20 inches for #4). Order 10% over for laps, offcuts and the bars that get cut around penetrations.