Slab rebar grid bar count and weight.

Slab rebar grid: how it works

Bars run both ways at the spacing you enter. Each bar stops short of the slab edge by the cover. The number of bars in one direction is the usable width divided by the spacing, rounded down, plus one. Each bar is the usable length of the other direction. Weight is total length times the bar's nominal linear mass: #3 0.376, #4 0.668, #5 1.043 and #6 1.502 lb/ft for US bars, and 10M 0.785, 15M 1.570, 20M 2.355 and 25M 3.925 kg/m for Canadian metric bars.

Worked example, imperial. A 20 ft by 12 ft slab, #4 bars at 12 in, 2 in cover, one mat, 10% allowance. Usable size is 19.67 ft by 11.67 ft. That gives 12 bars of 19.67 ft and 20 bars of 11.67 ft: 469.3 ft, or 516.3 ft with the allowance. At 0.668 lb/ft that is 344.9 lb.

Worked example, metric. A 6 m by 4 m slab, 15M bars at 300 mm, 50 mm cover, one mat, 10% allowance. Usable size is 5.9 m by 3.9 m. That gives 14 bars of 5.9 m and 20 bars of 3.9 m: 160.6 m, or 176.7 m with the allowance. At 1.570 kg/m that is 277.4 kg.

What this does not do. It counts bar length for a plain rectangular slab. It does not choose a bar size or spacing, size chairs or supports, or place extra bars at openings, thickened edges or footings. For the volume of concrete, use the Concrete Calculator.