Rebar calculator

Count the bars each way: take the slab width in inches, subtract the edge cover on both sides, divide by the bar spacing, round down and add one. Each of those bars runs the slab length minus the cover. Do the same the other direction and add the two totals. A 20 by 20 foot slab with bars 12 inches on center and 3 inches of cover takes 20 bars each way at 19.5 feet, 780 linear feet in all.

Standard weights are 0.376 pounds per foot for #3 bar, 0.668 for #4, 1.043 for #5 and 1.502 for #6. Multiply by the total linear feet for the order weight.

Lap splice length is set by the project engineer or the building code and depends on bar size, concrete strength and location. A common rule of thumb for simple slabs is around 40 times the bar diameter, which is 20 inches for #4 bar, but use the number on the plans.

Residential slabs and driveways are often reinforced with #3 or #4 bar at 12 to 18 inches on center each way, but spacing is a design decision. Follow the plans or your local code, especially for footings, structural slabs and anything carrying vehicles.

Bars need concrete cover so they do not rust. Slabs on grade commonly keep bars around 3 inches from the edges and supported on chairs or dobies so they sit in the slab rather than on the ground. The required cover comes from the plans and code.