Concrete & Masonry
Calculate rebar grid lines, lap splices, total linear feet, 20 ft stock bar counts, and steel weight for concrete slabs.
Enter slab length, width, on-center grid spacing, edge clearance, bar size, and stock stick length. The calculator determines exact runs, lap splices, and whole stock bars to order.
Calculation and worked example reviewed 2026-11-14. See our calculation methodology.
14 lengthwise + 14 widthwise runs (#4 rebar at 18" OC).
Grid ties: ~196 intersections · 0.2 US tons steel.
Formula and example
This estimator converts your entered jobsite inputs (slab length, slab width, rebar grid spacing (on-center), edge clearance / concrete cover, rebar size (astm standard), stock bar length, lap splice overlap, waste & cut allowance) into consistent US trade units, computes the exact geometric baseline first, and then applies any selected waste or order allowance so you can verify both numbers before purchasing materials.
where:
Net length = slab length − 2 × (edge clearance ÷ 12); net width = slab width − 2 × (edge clearance ÷ 12); lengthwise lines = floor(net width × 12 ÷ spacing) + 1; widthwise lines = floor(net length × 12 ÷ spacing) + 1; total linear ft = (lengthwise lines × run length with laps + widthwise lines × run width with laps) × (1 + waste% ÷ 100); stock bars = ceil(total linear ft ÷ stock length).
A 20 ft × 20 ft slab with 18 in grid spacing, 3 in edge clearance, #4 rebar, and 10% waste needs 14 lengthwise + 14 widthwise lines (546 base linear ft). With 10% waste, order 31 twenty-foot bars (601 linear ft, ~401 lbs).
Linear feet, 20 ft stock stick count, and steel weight for typical residential concrete slabs with 10% waste.
| Slab Size | Rebar Grid Lines | 20 ft Stock Bars | Total Steel Weight |
|---|---|---|---|
| 10 ft × 10 ft patio | 7 length + 7 width (14 runs) | 8 bars (20 ft) | 105 lbs (~0.05 tons) |
| 12 ft × 12 ft shed | 8 length + 8 width (16 runs) | 11 bars (20 ft) | 143 lbs (~0.07 tons) |
| 20 ft × 20 ft driveway | 14 length + 14 width (28 runs) | 31 bars (20 ft) | 401 lbs (~0.20 tons) |
| 24 ft × 24 ft garage | 16 length + 16 width (32 runs) | 44 bars (20 ft) | 583 lbs (~0.29 tons) |
Assumes a single-layer rectangular grid mat supported on rebar chairs. Edge beams, thickened footings, hooks, dowel breakouts, dual rebar mats, and structural seismic detailing require an engineering plan.
Standard residential concrete slabs, driveways, and patios typically use #4 rebar (1/2 inch diameter). Light patios or sidewalks may use #3 rebar (3/8 inch), while heavy-duty commercial slabs or foundation footings often require #5 rebar (5/8 inch).
Standard rebar grid spacing is 18 inches on-center (OC) in both directions. For heavier vehicle parking (e.g. RV pads or commercial driveways), spacing is typically tightened to 12 inches OC.
Under ACI 318 standards, rebar splices typically require an overlap of 30 to 40 bar diameters. For standard #4 rebar (0.5 inch diameter), 30 to 40 diameters equals 15 to 20 inches of overlap (18 inches is standard industry practice).
Standard ASTM #4 rebar (1/2 inch nominal diameter) weighs exactly 0.668 pounds per linear foot. A 20-foot stick of #4 rebar weighs approximately 13.36 pounds.
Method references: CRSI: Concrete Reinforcing Steel Institute field guide