Rebar Calculator
Rebar reinforces concrete and prevents cracking. This calculator estimates the number of rebar pieces needed for a grid layout in concrete slabs or footings.
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Results
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How to Use This Calculator
- Enter slab length and width in feet.
- Enter the desired rebar spacing (12 inches is common for residential slabs).
- Select the rebar size (#3 or #4 is typical for slabs).
- Review the quantity and weight estimates.
How Rebar Calculation Works
Rebar is laid in a grid pattern inside the concrete. This calculator determines how many bars run in each direction based on your spacing, then calculates total linear feet and weight. Standard practice is to place rebar at the center of the slab thickness, supported on rebar chairs. Bars should overlap by at least 40 bar diameters at splices (20 inches for #4 rebar).
Formula
Lengthwise bars = floor(Width × 12 ÷ Spacing) + 1. Widthwise bars = floor(Length × 12 ÷ Spacing) + 1. Total length = (Lengthwise × Length) + (Widthwise × Width). Weight = Total length × weight per foot for chosen size.
Frequently Asked Questions
What size rebar for a concrete slab?
#3 (3/8") or #4 (1/2") rebar is standard for residential slabs like driveways and patios. #5 (5/8") is used for structural footings, grade beams, and walls. Spacing is typically 12–18 inches on center.
Do I need rebar in a 4-inch slab?
For walkways and patios, wire mesh or fiber reinforcement is often sufficient. For driveways, garages, and any slab carrying loads, rebar is recommended. Some codes require it. When in doubt, use rebar — the cost is minimal compared to replacing a cracked slab.
How much does rebar cost?
#3 rebar costs about $0.15–$0.30 per linear foot. #4 runs $0.25–$0.50 per foot. A 20-foot stick of #4 rebar costs about $5–$10. Rebar chairs (supports) cost about $0.25–$0.50 each.