Bars each way, total length and weight for a slab or wall
Straight bars, both edge bars counted, lengths cut back by the cover. Add laps with the waste %.
Bar diameters, areas and weights are in the rebar sizes chart.
Subtract the cover from both edges, divide by the spacing, round down and add 1. A 10 ft wide slab with 3 in cover and 12 in spacing has 9.5 ft clear, so 9 + 1 = 10 bars running the long way.
Total length × unit weight. A #4 bar weighs 0.668 lb/ft and a #5 bar 1.043 lb/ft (ASTM A615). 385 ft of #4 weighs 385 × 0.668 = 257 lb.
The steel area per foot of width: bar area × 12 / spacing (in). #4 at 12 in gives 0.20 in²/ft; #5 at 12 in gives 0.31 in²/ft. In metric, bar area × 1000 / spacing (mm) gives mm²/m.
Bars are assumed continuous. If runs are longer than stock length (20 ft or 40 ft in the US, 12 m in metric), add the lap length per splice, or add 10 to 15 percent in the waste field as a quick estimate.
Slabs on grade often use #3 or #4 at 12 to 18 in each way. In structural slabs and walls, ACI 318 caps flexural bar spacing at the lesser of 3h and 18 in, and shrinkage and temperature steel at the lesser of 5h and 18 in.