Concrete Rebar Calculator

Most residential slabs (driveways, patios, garage floors) use #4 rebar (1/2" diameter) at 16" on-center grid spacing. Formula: bars one way = floor(width / spacing-in-feet) + 1, bars the other way = floor(length / spacing-in-feet) + 1, then total linear feet = (bars across width × length) + (bars across length × width), plus 10% for laps and corners. A 20×24 ft driveway = ~840 lf (10% lap-and-corner waste included) = 43 sticks of 20-ft #4. At $0.55-0.75/lf in 2026, that's roughly $460-$630 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, 3" where concrete is cast directly against earth (such as footings poured into an unformed trench), 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.

For light residential slabs like small patios, 6x6 welded wire mesh is an acceptable, cheaper alternative. For heavy loads, driveway transitions, or slabs thicker than 4 inches, tied rebar in a grid performs much better structurally.

Weight depends on the bar size: #3 weighs 0.376 lb per foot, #4 weighs 0.668, #5 weighs 1.043, and #6 weighs 1.502 lb per foot. A standard 20-foot #4 stick weighs about 13.4 lb.

You don't need to tie them all. Common practice is to tie every second or third intersection in a staggered pattern, which is enough to hold the grid firm while the concrete is poured. This tool counts every intersection so you have an upper bound.