What rebar size should I use for a concrete slab?

#3 rebar (3/8" diameter) is used for sidewalks and light residential slabs. #4 rebar (1/2" diameter) is the standard for driveways, garage floors, and residential slabs bearing vehicle loads. #5 rebar (5/8" diameter) is used for heavy-duty slabs, retaining walls, and structural applications.

How far apart should rebar be spaced?

12-inch on-center spacing is standard for most residential concrete slabs including driveways and patios. 18-inch spacing is sometimes used for lightly loaded interior slabs. Closer spacing (8–10 inches) is used for structural slabs or areas with higher loads. Always check your local building code or engineering specifications.

How deep should rebar be placed in concrete?

Rebar should sit in the lower third of the slab, approximately 1.5 to 2 inches from the bottom. Use plastic rebar chairs to hold bars at the correct height before pouring. Rebar too close to the surface can rust and cause concrete to spall. Rebar too deep reduces its effectiveness at controlling tensile cracking.

What is the weight of rebar per linear foot?

Standard rebar weights per linear foot: #3 (3/8") = 0.376 lb/ft, #4 (1/2") = 0.668 lb/ft, #5 (5/8") = 1.043 lb/ft, #6 (3/4") = 1.502 lb/ft. These weights help estimate shipping costs and determine how many bundles you need — rebar is typically sold in 20-foot lengths bundled at the lumber yard.

Does a sidewalk need rebar?

Sidewalks typically do not require rebar under residential building codes. Wire mesh (6×6-W1.4×W1.4 welded wire reinforcement) is sufficient for a 4-inch sidewalk. However, if your sidewalk crosses a driveway, is near a tree with invasive roots, or is in a freeze-thaw climate with poor soil, adding #3 rebar at 18-inch spacing dramatically reduces cracking.

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Rebar Calculator

Calculate rebar quantity, linear footage, weight, and cost for any concrete project.

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How to Calculate Rebar for a Concrete Slab

The rebar calculator figures out how many reinforcing bars you need in both directions, accounting for spacing, overlap at splices, and the standard 20-foot bar length sold at most suppliers. Here is how the math works.

Grid Pattern Calculation

Rebar is placed in a grid pattern — bars running in the lengthwise direction, bars running in the widthwise direction, spaced evenly. For a 20×10 slab with 12-inch spacing: widthwise bars = floor(20 ÷ 1) + 1 = 21 bars, each 10 ft long = 210 linear ft. Lengthwise bars = floor(10 ÷ 1) + 1 = 11 bars, each 20 ft long = 220 linear ft. Total = 430 linear ft.

Divide total linear feet by 20 (standard bar length) to get the number of bars. Allow 5–10% extra for cutting waste and end placement. For 430 ft: 430 ÷ 20 = 21.5 bars, round up to 22 bars minimum, buy 24 for safety.

Rebar Size Guide

#3 (3/8" diameter, 0.376 lb/ft): Sidewalks, lightweight patios, not for vehicle loads. #4 (1/2" diameter, 0.668 lb/ft): Standard for driveways, garage floors, and residential slabs with vehicle access. #5 (5/8" diameter, 1.043 lb/ft): Retaining walls, structural slabs, commercial applications. #6 and larger: Engineering specification required — consult a structural engineer.

Placement Depth

Rebar must be placed in the lower third of the slab — 1.5 to 2 inches from the bottom for a 4-inch slab, 2 to 2.5 inches from the bottom for a 6-inch slab. Use plastic rebar chairs (available at any hardware store) to hold bars at the correct elevation before pouring. Never use wood chips, rocks, or scraps — they rot, move, or telegraph through the concrete surface.

Rebar vs. wire mesh: Wire mesh (6×6-W1.4×W1.4) is cheaper and faster to install but provides less reinforcement. For driveways and any slab with vehicle traffic, use rebar. Wire mesh is adequate for patios and interior floors not subjected to vehicle loads.

Tying Rebar

Bars must be tied at every intersection using rebar wire ties. Use a rebar twister tool (hook tool) to twist wire ties quickly. Overlapping bars at splices (where a 20-ft bar ends and another begins) must overlap at least 40 times the bar diameter — 20 inches for #4 rebar. This is why the 6-inch overlap input in our calculator adds to total footage.

Cost Estimate

Rebar pricing varies significantly by market and season. Typical retail prices: #3 rebar = $0.30–$0.50/ft, #4 = $0.45–$0.75/ft, #5 = $0.70–$1.10/ft. A 20-ft length of #4 rebar costs $9–$15 at hardware stores. Buying in bulk from a steel supplier saves 20–40% on large projects. Add $15–$25 for rebar chairs, wire ties, and a twist tool.

Frequently Asked Questions

#3 for sidewalks and light patios, #4 for driveways and garage floors, #5 for structural or heavy-duty applications. When in doubt, use #4 — it handles most residential needs.

12-inch on-center spacing is standard for residential slabs. 18 inches for lightly loaded interior floors. 8–10 inches for heavy structural slabs. Always check local codes or engineering specs.

1.5–2 inches from the bottom for a 4-inch slab. Always use plastic rebar chairs — never wood or rocks — to hold bars at the correct depth before pouring.

#3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft. Weight matters for delivery — a full truckload of #4 rebar for a large driveway can weigh 2,000+ pounds.

Building codes typically allow wire mesh for sidewalks. However, rebar dramatically reduces cracking in freeze-thaw climates, near trees, or anywhere soil conditions are poor. The cost difference is small for the benefit.

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