Rebar Calculator

Rebar (reinforcing bar) is a steel bar or mesh used to reinforce concrete structures. Concrete is strong in compression but weak in tension, while steel is strong in tension. Together, they create reinforced concrete that can withstand both compressive and tensile forces, making structures stronger and more durable.

To calculate rebar needed: (1) Determine slab dimensions, (2) Choose your rebar spacing (typically 12-18 inches on center), (3) Calculate bars along length = (width ÷ spacing) + 1, (4) Calculate bars along width = (length ÷ spacing) + 1, (5) Multiply number of bars by slab dimensions to get linear feet, (6) Add 10% waste factor.

Standard rebar spacing for residential concrete slabs is typically 12 to 18 inches on center in both directions, forming a grid pattern. However, spacing can vary based on load requirements, slab thickness, and local building codes. Heavy-duty applications may require closer spacing (8-12 inches), while light-duty slabs might use wider spacing (18-24 inches).

For typical residential slabs (driveways, patios, garage floors), #3 or #4 rebar is most common. #3 (3/8" diameter) is suitable for lighter applications like sidewalks, while #4 (1/2" diameter) is better for driveways and garage slabs. Commercial applications or heavily loaded areas may require #5 or larger. Always consult local building codes and a structural engineer for specific requirements.

Rebar weight varies by size: #3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft, #7 = 2.044 lb/ft, #8 = 2.670 lb/ft. Knowing the weight helps estimate transportation costs and ensures your vehicle or equipment can handle the load.

Rebar should typically be placed in the lower third of the slab thickness, about 2-3 inches from the bottom for a 4-6 inch slab. This positioning helps resist tensile stresses that occur when the slab bends under load. Use rebar chairs or supports to maintain proper spacing from the ground during concrete placement.