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.