Rebar Calculator

Structural Concrete Reinforcement & ACI 318 Specifications

Reinforcing bar provides the critical tensile and shear strength necessary to prevent plain concrete from cracking under flexural tension, thermal shrinkage, and soil settlement loads.

Core Rebar Calculation Equations

• Effective Grid Length: L_eff = L_slab - 2 × (Edge Cover (in)12)

• Number of Longitudinal Lines: N_long = W_effS_grid + 1

• Number of Transverse Lines: N_trans = L_effS_grid + 1

• Total Linear Footage: F_total = - Total 20-Foot Sticks: N_sticks = F_total20

• Total Steel Weight: W_lbs = F_total × w_bar

Standard US Rebar Sizes & Weights

• #3: 0.376 lbs/ft (patios, sidewalk crack control)

• #4: 0.668 lbs/ft (residential slabs, garage foundations, retaining walls)

• #5: 1.043 lbs/ft (commercial grade slabs, heavy equipment pads)

• #6: 1.502 lbs/ft (structural footing cages, bridge abutments)

American Concrete Institute (ACI) Rules of Thumb

• Clearance / Cover: Maintain at least 3 inches cover against earth and 1.5 inches to 2 inches cover inside formwork.

• Lap Splice: Minimum lap overlap should equal at least 30 to 40 bar diameters (24 inches for #4 rebar).

How to Use This Calculator

Enter your Slab / Footing Length (e.g. 24.0 in ft), Slab / Footing Width (e.g. 20.0 in ft), US Standard Rebar Size — choose from #3 Bar (3/8" - 0.376 lbs/ft - Driveways & Patios), #4 Bar (1/2" - 0.668 lbs/ft - Standard Slabs & Footings), #5 Bar (5/8" - 1.043 lbs/ft - Commercial / Structural), Grid Spacing (On-Center) — choose from 12" O.C. Grid (Heavy Load / Commercial), 16" O.C. Grid (Intermediate Reinforcement), 18" O.C. Grid (Standard Residential Slab), Concrete Edge Cover Clearance (e.g. 3.0 in in), Lap Splice Overlap (e.g. 24.0 in in), and Waste & Cutting Allowance (%) (e.g. 10.0 in %). Measure twice before entering dimensions — small measurement errors scale up significantly in material estimates.

The Formula

Wire ties: approx 1 tie per intersection; 1 lb of 16-ga tie wire = ~200 ties

Worked Example

Reinforcing a 24'x20' concrete driveway slab with #4 rebar on an 18" grid: Requires 14 long lines and 16 cross lines, totaling 654 linear feet (33 twenty-foot sticks weighing 437 lbs).

Understanding Your Result

Construction estimates are starting points — site conditions, material specifications, and local code requirements will affect final quantities and costs. Always obtain at least three contractor bids for major projects. Local building departments can confirm permit requirements and inspection milestones. Add waste factors appropriate to the material type and complexity: 10% for simple rectangular shapes, 15–20% for complex geometries or custom cuts.