Rebar Calculator — Slab Grid & Linear Feet

A reinforced slab uses two sets of parallel bars crossing at right angles. Count each direction on its own: divide the span perpendicular to the bars by the spacing, add one bar to close the run, and multiply by the length each bar has to travel. For a 20 by 20 foot slab at 18 inches on center, that is 15 bars running each way at 20 feet apiece, or 600 total linear feet before laps.

Spacing follows the load. Eighteen inches on center with #4 bar is the common residential slab-on-grade layout. Twelve inches on center, or a step up to #5 bar, suits driveways carrying heavy vehicles, garage floors, and slabs on expansive clay soils. Footings are counted differently — they use continuous longitudinal bars with transverse ties rather than a grid, so measure those runs separately from the slab field.

Rebar sticks are sold in 20 foot lengths, so any run longer than 20 feet needs a lapped splice. A standard residential lap for #4 bar is about 25 inches, roughly 40 bar diameters, wired tight at two points. The 10% allowance built into this calculator covers typical lapping on a residential slab; large commercial pours with many splices need the laps counted explicitly.

Position matters as much as quantity. Reinforcement placed on the ground does nothing — it must sit in the middle third of the slab depth, with a minimum of 3 inches of cover where concrete is cast against earth and 1-1/2 to 2 inches elsewhere. Use rebar chairs or dobies at 3 to 4 feet in each direction. Hooking the grid up with a rake during the pour, sometimes called wet-setting, is unreliable and should not be planned for.

In 2026, #4 grade 60 rebar averages about $0.75 per linear foot in the USA, which is around $15 for a 20 foot stick. #3 bar runs closer to $0.48 a foot and #5 near $1.15. Welded wire mesh in 5 by 10 foot sheets costs roughly $1.10 per square foot and is a legitimate substitute for light, unloaded slabs such as small patios and shed pads.

For anything carrying vehicles, rebar is the better buy. Mesh is thinner, easier to walk flat during the pour, and gives less crack control once it ends up at the bottom of the slab. Add fiber to the mix if you want plastic-shrinkage crack control, but understand that fiber supplements reinforcement, it does not replace the structural steel that a driveway or a footing requires.