Concrete Reinforcement Calculator

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. The steel only works if the surrounding concrete reaches its design strength, so match the bar layout to a mix that fits the application — the standard residential mixes by application walks through which proportions belong in slabs versus footings versus walls. After placing the steel, allow adequate curing time before loading the slab — rebar holds cracks tight but cannot compensate for concrete that was loaded too early.

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 for #4 bar and tie the lap splice with three wire ties: 40 bar diameters is the rule of thumb, and a Class B tension splice in 4,000 psi concrete works out near 50 diameters (25 inches for #4, 31 inches for #5). Use the rebar lap splice calculator to find the correct overlap for your bar size, concrete strength and cover.

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. The calculator counts bars and cuts bar lengths inside that 3-inch cover, so the outer bars never sit on the form.

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 2 to 3 foot intervals, the common practice the Wire Reinforcement Institute prints for reinforcement supports (TF 202-R-18); this page formerly said roughly 4 feet. Steel sitting on the subgrade does nothing structural and corrodes rapidly.

Prices and rates on this page are our own estimates as of the date shown; they change often. Enter your own figures for a current result. This table covers the sizes common in residential slabs; the full chart of bar sizes from #3 to #18 adds the larger structural bars and their soft-metric equivalents. 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. For slabs in fire-rated assemblies, check fireproofing material requirements — concrete cover over rebar doubles as fire protection in many building code tables.

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 heavy 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.

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

The four tiers are ACI 318-19 Table 20.5.1.3.1, and the IRC prints the same values in R403.1.3.5.3 for footings and R404.1.3.3.7.4 for foundation walls. Cited to IRC R403.1.3.5.3 and R404.1.3.3.7.4. The code text itself is not freely available to link; confirm the figure against the current edition or your local building department before acting. Municipal renderings of the same tiers are on record (Seattle's Residential Code chapter 4, Loudoun County's cast-in-place concrete specification, the New York City Building Code's chapter 19 pointer to ACI 318).

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, 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 can help if your footing supports a steel column — the footing dimensions and reinforcement must match the column load.