Concrete Reinforcement Calculator

Getting Rebar Quantities Right Before the Pour

Running short on rebar mid-pour is one of those mistakes you only make once. Concrete waits for nobody — the truck is on the clock, the crew is on the clock, and pausing to send someone for another bundle of #4 bar costs time, money, and quality. Over-ordering is almost as bad. Rebar is heavy (a bundle of 20-foot #4 bars weighs about 267 pounds), takes up space on site, and leftover stock is hard to return.

This calculator gives you the exact piece count and total linear footage for a rectangular grid of rebar in a slab or footing. You enter the slab dimensions, choose the bar size and spacing, and get back the number of bars in each direction, total weight, tie wire needed, and an estimated material cost.

The standard approach for slab-on-grade reinforcement is a grid pattern: bars running in both directions at equal spacing, tied together at every intersection with wire ties. This creates a mesh that distributes tensile forces from shrinkage cracking, temperature changes, and point loads. For most residential slabs — driveways, garage floors, patios — #4 bar at 12-inch spacing in both directions exceeds code minimums and provides good crack control.

How to Place Rebar in a Concrete Slab

1. **Set the edge forms and compact the subgrade.** Rebar placement starts after the forms are built and the gravel base is compacted and graded. Any soft spots in the subgrade will cause differential settlement regardless of how much steel you put in the concrete.

2. **Cut rebar to length.** Standard rebar comes in 20-foot lengths. For a 20-foot slab, bars span the full length. For slabs longer than 20 feet, overlap bars by at least 24 inches (40 bar diameters for #4) and tie the lap splice with three wire ties.

3. **Lay the first direction.** Place all bars running in one direction at the specified spacing. Use a tape measure from the form edge and mark spacing increments. Maintain 3 inches of clear cover from the slab edges — this keeps the steel from corroding where moisture can reach it.

4. **Lay the perpendicular direction on top.** Place the second layer of bars at right angles to the first, using the same spacing. The second layer sits on top of the first.

5. **Tie intersections.** Wrap 16-gauge tie wire around every intersection or every other intersection (for slabs, tying every other one is acceptable — the concrete locks everything in place). Use a tie wire reel and a simple hook tool to speed this up.

6. **Set bar chairs.** Rebar must sit at the correct height within the slab — not on the ground. For a 4-inch slab, position the steel at mid-depth (2 inches from the bottom) using plastic or wire bar chairs (supports) at roughly 4-foot intervals. Steel sitting on the subgrade does nothing structural and corrodes rapidly.

Rebar Sizes and Properties

| Bar Size | Diameter | Weight (lbs/ft) | Cross-Section Area (in²) | Typical Use | Cost/ft (March 2026) | |---|---|---|---|---|---| | #3 | 3/8" (9.5 mm) | 0.376 | 0.11 | Light slabs, temperature steel, ties | $0.30–$0.45 | | #4 | 1/2" (12.7 mm) | 0.668 | 0.20 | Standard slabs, footings, walls | $0.45–$0.65 | | #5 | 5/8" (15.9 mm) | 1.043 | 0.31 | Heavy footings, retaining walls, grade beams | $0.70–$1.00 | | #6 | 3/4" (19.1 mm) | 1.502 | 0.44 | Foundation walls, columns, heavy structural | $1.00–$1.40 | | #7 | 7/8" (22.2 mm) | 2.044 | 0.60 | Commercial foundations, bridge decks | $1.35–$1.85 |

Prices as of March 2026, US national averages for Grade 60 rebar. Epoxy-coated rebar (green) adds 25–40% for corrosion protection in exposed or marine environments. Stainless steel rebar costs 5–8x standard but eliminates corrosion entirely.

Rebar vs Welded Wire Mesh: When to Use Each

**When should I use rebar instead of welded wire mesh?** Rebar is the better choice for structural applications: footings, grade beams, retaining walls, and any slab thicker than 5 inches. Rebar provides defined bar sizes and controlled spacing, and it stays in position during the pour when properly chaired. For slabs that carry significant loads — garage floors with heavy vehicles, shop floors with equipment — rebar gives you more confidence in the reinforcement placement.

**When is welded wire mesh acceptable?** Welded wire mesh (WWM) works well for lightly loaded slabs-on-grade: sidewalks, patios, light-duty residential driveways, and interior floor slabs. Common mesh sizes are 6x6-W1.4/W1.4 (6-inch grid of light wire) and 6x6-W2.9/W2.9. Mesh is faster to lay than individual rebar — you unroll a sheet and cut to size. The drawback is that mesh tends to end up on the bottom of the slab during the pour (workers step on it and it sinks into the wet concrete), which makes it structurally useless unless it stays elevated on chairs throughout the pour.

**Can I combine rebar and mesh?** Yes. A common hybrid approach for driveways and garage floors uses #4 rebar at 16–18 inch spacing in both directions supplemented by a layer of wire mesh. The rebar carries the structural load and the mesh provides tighter crack control between the bars. This costs more but gives you the positioning reliability of rebar with the crack control of mesh.

Concrete Cover and Corrosion Protection

Rebar rusts. When it does, the iron oxide (rust) expands to roughly six times the volume of the original steel, which cracks the surrounding concrete from the inside out. Those spider-web cracks you see on old sidewalks and bridge decks? That is rebar corrosion at work.

The primary defence against corrosion is concrete cover — the thickness of concrete between the rebar and the nearest exposed surface. ACI 318 (the governing US concrete code) specifies minimum cover requirements based on exposure conditions:

- Concrete cast against and permanently in contact with earth: 3 inches minimum - Concrete exposed to weather (#6 bars and larger): 2 inches - Concrete exposed to weather (#5 bars and smaller): 1.5 inches - Concrete not exposed to weather or in contact with ground: 0.75 inches

For a typical slab-on-grade — a garage floor or basement slab — the bottom of the slab contacts the earth, so the bottom cover should be 3 inches. The top surface is exposed to weather (freeze-thaw cycles, de-icing salts on a driveway), so the top cover needs 1.5–2 inches.

In a 4-inch slab, that leaves very little room to position the steel correctly — the bar essentially needs to sit at mid-depth. This is why bar chairs are non-negotiable. If rebar ends up on the subgrade with zero bottom cover, it will corrode and the slab will crack within 10–15 years.

If your concrete project involves structural elements like [block walls or footings](/calculators/structural/concrete-block-wall-cost-calculator), the rebar requirements and cover rules differ — block wall reinforcement uses vertical bars grouted into the block cores, where cover is provided by the block shell rather than poured concrete. The [steel beam calculator](/calculators/structural/steel-beam-size-calculator) can help if your footing supports a steel column — the footing dimensions and reinforcement must match the column load.

Worked Examples