Rebar Spacing Calculator

Calculate the number of rebar pieces, grid layout, and total linear feet needed for your concrete slab. Works for any slab size and spacing requirement.

How to Calculate Rebar for a Slab

Rebar is laid in a grid pattern. The number of bars in each direction depends on the slab dimensions and the spacing between bars. For a 20-foot slab with 16-inch spacing: 20 feet = 240 inches ÷ 16 = 15 bars running one direction, plus 1 for the edge = 16 bars.

Standard overlap for #4 rebar is 18 inches at each splice. If your slab is longer than the standard rebar length (usually 20 feet), you'll need to account for extra overlap.

Standard Rebar Spacing by Application

Residential Slabs (Patios, Driveways)

#4 rebar at 16–18 inches on center is standard. For lighter loads (patios, walkways), #3 at 18–24 inches is often sufficient.

Structural Slabs

#5 or #6 rebar at 12 inches on center is common for foundations and structural slabs. Engineering specifications should always be followed for load-bearing applications.

Footings

Continuous rebar (2–4 bars depending on width) running the length of the footing, with cross bars at 8–12 inches on center.

Frequently Asked Questions

What size rebar for a 4-inch slab?

#3 or #4 rebar is standard for a 4-inch residential slab. #3 at 18–24 inches on center for patios and walkways; #4 at 16 inches for driveways and heavier use.

Can I use wire mesh instead of rebar?

Wire mesh works for light-duty slabs (patios, sidewalks) but doesn't provide the same structural reinforcement. For driveways and anything with vehicle traffic, rebar is recommended.

How much does rebar cost?

#4 rebar costs roughly $0.50–$0.90 per linear foot at retail. A 20×20 slab at 16-inch spacing requires about 250 linear feet, or $125–$225 in materials.