Concrete Rebar Calculator

Most residential slabs (driveways, patios, garage floors) use #4 rebar (1/2" diameter) at 16" on-center grid spacing. Formula: total linear feet = (length × 12 / 16) × width + (width × 12 / 16) × length, then convert to 20-foot lengths. A 20×24 ft driveway = ~360 lf = 18 sticks of 20-ft #4. Add 10% for lap splices and corners. At $0.55-0.75/lf in 2026, that's $200-$270 in rebar.

#3 (3/8") - light residential slabs, sidewalks. #4 (1/2") - most residential: driveways, patios, garage floors, footings. #5 (5/8") - heavy-duty residential, basements, retaining walls. #6 (3/4") - commercial or thick load-bearing pours. ACI recommends consulting an engineer for any structural application. Welded wire fabric (WWF) is an alternative for thin slabs (4-5") but doesn't replace rebar for load-bearing applications.

Per ACI 318: 1.5" minimum for slabs on grade (cast against earth), 3" for footings cast against earth, 2" for above-grade vertical surfaces exposed to weather, 1.5" for interior dry. Cover is critical - too little allows corrosion (rebar rust expands 7× and cracks concrete); too much reduces structural effectiveness. Use rebar chairs (plastic or metal) every 3-4 ft to maintain spacing during pour.

Standard formula: 40× the rebar diameter for tension splices in #4 rebar = 20" lap. Bigger bars need bigger laps: #5 = 25", #6 = 30". For corners and bends, add 10% to your linear feet estimate. Always tie laps with 16-gauge tie wire (1 lb per ~150 ties). Welded splices are stronger but rarely cost-effective for residential. Mechanical couplers exist but typically only on commercial projects.