How much rebar do I need for a slab?
Rebar reinforces a slab as a grid: bars running one way, crossed by bars running the other way, tied where they meet. To size it, count the bars in each direction, add up their lengths, then convert to the number of stock lengths you'll buy. The formula this calculator uses is:
Bars one way = (dimension − 2 × clearance) ÷ spacing, rounded down, + 1
Each lengthwise bar spans the slab's length minus the edge clearance, and each widthwise bar spans the width minus clearance. We total both sets to get linear feet, then divide into your stock lengths (commonly 20 ft) and round up. A typical 20 × 20 ft slab on a 16-inch grid works out to about 15 bars each way — roughly 585 ft, or 30 lengths.
Rebar size and spacing by project
Heavier loads call for thicker bar and a tighter grid. These are common residential starting points — always defer to your local code or an engineer's plan for anything structural.
| Project | Rebar size | Spacing |
|---|---|---|
| Patio, walkway, shed floor | #3 (3/8 in) | 16–24 in |
| Driveway, garage floor, slab | #4 (1/2 in) | 12–18 in |
| Footings, heavy / structural slab | #5 (5/8 in)+ | 12 in or per plan |
For reference, rebar weighs about 0.376 lb/ft for #3, 0.668 lb/ft for #4, and 1.043 lb/ft for #5 — handy when you're checking what will fit in your vehicle.
Placing and tying rebar
- Lift it off the ground. Rebar belongs in the lower-middle third of the slab. Set it on chairs or dobies — bars lying on the subgrade do almost nothing.
- Keep your cover. Leave about 3 in from the edges and 2 in of concrete below the bar so it can't rust.
- Overlap splices. Where bars meet end-to-end, lap them about 40 bar diameters (at least 12 in) and tie them — add this to your total if your runs are longer than a stock length.
- Tie, don't weld. Secure intersections with tie wire; welding weakens standard rebar.
Rebar vs. wire mesh
Both control cracking, but they're built for different loads. Welded wire mesh (or fiber mixed into the concrete) is enough for light-duty slabs like a shed floor, a thin patio, or a sidewalk. Rebar is stronger and the right call for anything that carries real weight — driveways, garage floors, footings, and structural slabs — because it resists bending, not just surface cracking. For a heavily loaded slab you can use both: a rebar grid for strength plus mesh near the surface for crack control. When in doubt, size up to rebar.
Buying and cutting rebar
Rebar is sold in stock lengths — 10, 20, and sometimes 40 feet are common, with 20 ft the usual home-center length. Set your stock length in the calculator so the "lengths to buy" matches what's on the rack. Cut it with a rebar cutter, bolt cutters, or an angle grinder with a metal cut-off wheel, and bend lighter bars (#3–#4) over a sturdy edge or with a hickey bar. Wear gloves and eye protection — cut ends are sharp — and cap any vertical bars so no one falls onto them.