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The grid count, the weight, the stick order — before the pour.
Rebar is priced by weight and sold in 20-foot sticks, so a slab order needs three numbers: bars each way, total linear feet with lap allowance, and pounds at your bar size. This calculator runs the grid math (⌈dimension ÷ spacing⌉ + 1 per direction), applies ASTM nominal weights (#4 = 0.668 lb/ft), and converts to sticks and cost.
Anyone pouring a slab uses this the day before the steel order: the grid count is easy, but the laps allowance and the pounds-to-sticks conversion are where orders come up short. Concrete subs use the weight output as a quick check against the supplier quote.
At 18" on center with #4 bar: 17 bars each way = 816 linear feet, about 881 LF with 8% for laps and trim, which weighs ~589 lb — roughly 45 twenty-foot sticks. A 12" grid for a driveway of the same size would be 25 bars each way.
Rebar for driveways, structural slabs, and anything over 4–5 inches; wire mesh is adequate for light 4" patios and sidewalks. Either way, the steel must sit at mid-depth on chairs — steel lying on the ground adds almost no strength.