Rebar Calculator

Calculating Rebar Requirements for Concrete Reinforcement

Rebar (reinforcing bar) provides tensile strength to concrete, which is strong in compression but weak in tension. This calculator determines the number of rebar pieces, total linear feet, and weight needed for your concrete project based on slab dimensions, bar spacing, and bar size. Standard rebar spacing for residential slabs is 12 to 18 inches on center in a grid pattern, with #4 (1/2-inch diameter) bars being the most common choice for driveways, patios, and garage slabs.

Rebar is designated by numbers corresponding to its diameter in eighths of an inch: #3 rebar is 3/8 inch (0.376 lb/ft), #4 is 1/2 inch (0.668 lb/ft), #5 is 5/8 inch (1.043 lb/ft), and #6 is 3/4 inch (1.502 lb/ft). Where bars overlap, a lap splice of at least 24 bar diameters is required -- for #4 rebar, that is 12 inches minimum, though 24 inches is common practice. Standard rebar comes in 20-foot lengths, so factor in the number of splices needed for runs longer than 20 feet and add the splice length to your total.

Proper rebar placement is as important as quantity. Bars should sit at approximately one-third of the slab depth from the bottom for ground-supported slabs, supported by rebar chairs or concrete bricks rather than resting on the subgrade. For elevated slabs and beams, both top and bottom reinforcement is needed. In freeze-thaw climates, maintain at least 2 inches of concrete cover over the rebar to protect against corrosion from deicing chemicals. For any structural application like retaining walls, foundations, or second-story floors, always follow an engineer's reinforcement design rather than relying on general guidelines.