Rebar Calculator
Calculate reinforcing steel (rebar) grid requirements for concrete slabs, footings, and driveways. Computes linear feet, steel weight, 20ft stock bars, tie wire, and support chairs.
Calculation Results
Overall linear feet of reinforcing steel needed including lap splice overlaps and waste.
Total number of standard commercial 20ft rebar sticks to purchase.
Total shipment weight of steel reinforcement.
Support stands (dobies/chairs) placed every 2.5 to 3 feet to elevate rebar off the ground.
Rolls of 16-gauge black annealed tie wire for tying bar intersections.
How This Calculator Works
Plan your concrete reinforcement grid with these contractor steps:
- Enter Slab Dimensions: Input length and width of the concrete form.
- Select Rebar Size: #4 (1/2" diameter) is standard for residential driveways and garage floors; #3 (3/8") is common for sidewalks and patios; #5 (5/8") is used for commercial or foundation footings.
- Set Grid Spacing: 16-inch or 18-inch on-center grids are typical for residential slabs.
- Review Bill of Materials: Get exact 20ft stock bar counts, total steel poundage, tie wire rolls, and rebar support chairs.
Formula & Calculations
Rebar grid estimation follows American Concrete Institute (ACI 318) structural guidelines for concrete slab crack control:
1. Grid Bar Counts
- Longitudinal Bars (Along Length):
Count = ceil((Slab Width - 2 × Edge Clearance) / Spacing) + 1 - Transverse Bars (Along Width):
Count = ceil((Slab Length - 2 × Edge Clearance) / Spacing) + 1
2. Lap Splicing Overlaps (ACI Standards)
Because rebar is shipped in 20-foot lengths, longer spans require overlapping adjacent bars. The minimum structural lap splice is 30 to 40 bar diameters:
- #3 Rebar (3/8"): Minimum 15-inch lap overlap.
- #4 Rebar (1/2"): Minimum 20-inch lap overlap.
- #5 Rebar (5/8"): Minimum 25-inch lap overlap.
3. Elevation & Support Chairs
Rebar must never sit on the dirt or gravel subgrade. Plastic or concrete chairs (dobies) must support the steel grid in the upper or middle third of the slab, spaced roughly 2.5 to 3 feet on-center (approx. 1 chair per 7.5 sq ft of slab area).
Step-by-Step Calculation Examples
Two-Car Garage Slab (24ft x 20ft, #4 Rebar @ 18in OC)
Reinforcing a standard 480 sq ft garage floor slab with a single grid, 3in edge cover, and 10% overlap waste.
- length: 24
- width: 20
- spacing: 18
- rebar_size: 0.668
- clearance: 3
- grid_layers: 1
- waste: 10
- total_rebar_length: 747.2 ft
- bars_needed: 38
- total_weight: 499.1 lb
- support_chairs: 64
- wire_rolls: 1
Frequently Asked Questions
What size rebar is best for a concrete driveway or patio?
#4 rebar (1/2-inch diameter) is the industry standard for residential driveways, garage floors, and building slabs. For light-duty walkways or backyard patios, #3 rebar (3/8-inch) is also commonly used.
How far apart should rebar be spaced in a concrete slab?
Standard grid spacing is 12 to 18 inches on center in both directions. An 18-inch grid is standard for patios and shed pads, while a 12-inch or 16-inch grid is recommended for vehicular traffic driveways.
Why do I need rebar support chairs (dobies)?
Rebar only provides tensile reinforcement if it is encased in the center or upper third of the concrete slab. If rebar rests on the ground during the pour, it offers zero structural support and will quickly rust from ground moisture.
How much overlap is required when splicing rebar?
ACI building codes require a lap splice overlap equal to at least 30 to 40 times the bar diameter. For standard #4 (1/2") rebar, overlap adjoining bars by at least 20 inches and secure them with tie wire.
What does "edge clearance" or "concrete cover" mean?
Edge clearance is the space between the ends of the rebar grid and the wooden concrete forms (typically 2 to 3 inches). Maintaining this buffer prevents moisture from reaching the steel ends and causing rust-jacking and concrete spalling.
Can I use wire mesh instead of rebar?
Welded wire mesh (WWM) is suitable for light pedestrian sidewalks and patios, but rebar provides vastly superior structural load-bearing capacity and crack control for driveways, garage floors, and heavy equipment pads.