ft
Enter lengthft
Enter widthMost slabs use grid layout (both ways)
$
Check current prices at your supplier
Total Rebar Weight
—
🔩 Note: This calculator estimates rebar for a flat slab or rectangular element. For complex shapes, footings, columns, or beams, calculate each section separately. Always verify rebar size and spacing requirements with your structural engineer or local building code.
Was this calculator helpful?
How to Calculate Reinforcing Steel (Rebar)
Rebar (reinforcing steel) is embedded in concrete to add tensile strength, preventing cracking under load. The amount needed depends on the area being reinforced, rebar spacing, bar size, and whether you're running bars in one or both directions.
Rebar Calculation Formula
Bars in one direction = (Length ÷ Spacing) + 1, × Width for total linear feet
Total Linear Feet = ((Length / Spacing + 1) × Width + (Width / Spacing + 1) × Length) × directions
Add 10% for waste and lap splices.
Example: 20×15 ft slab, #4 rebar, 12" spacing, grid:
Bars L→W: (20/1+1)×15 = 315 ft | Bars W→L: (15/1+1)×20 = 320 ft
Total = 635 ft × 1.10 = 699 linear ft → 467 lbs → ~$314
Add 10% for waste and lap splices.
Example: 20×15 ft slab, #4 rebar, 12" spacing, grid:
Bars L→W: (20/1+1)×15 = 315 ft | Bars W→L: (15/1+1)×20 = 320 ft
Total = 635 ft × 1.10 = 699 linear ft → 467 lbs → ~$314
Standard Rebar Sizes & Weights
- #3 (3/8"): 0.376 lb/ft — light-duty: walkways, small slabs
- #4 (1/2"): 0.668 lb/ft — residential slabs, driveways, footings
- #5 (5/8"): 1.043 lb/ft — structural slabs, retaining walls, beams
- #6 (3/4"): 1.502 lb/ft — heavy structural applications
- #7 (7/8"): 2.044 lb/ft — columns, heavy beams
- #8 (1"): 2.670 lb/ft — bridge decks, heavy structural
Rebar Spacing Guidelines
- 6" spacing: Heavy structural slabs, seismic zones, high-load applications
- 12" spacing: Standard residential slabs, driveways, patios
- 18" spacing: Light-duty residential, decorative slabs
💡 Pro Tip: Always add a minimum 3-inch concrete cover over rebar to protect it from corrosion. Use rebar chairs or supports to maintain proper position. Lap splices should overlap 40× the bar diameter minimum.
Popular Calculators
Related Calculators
Frequently Asked Questions
How do I calculate how much rebar I need?
For a concrete slab: divide the slab length by the rebar spacing to get the number of bars in one direction, then multiply by the slab width for total linear feet in one direction. Double for both directions (grid). Add 10% for waste and overlaps. Multiply by the rebar weight per linear foot for total weight.
What size rebar should I use for a concrete slab?
#3 rebar (3/8 inch) at 18-inch spacing is standard for residential slabs up to 4 inches thick. #4 rebar (1/2 inch) at 12-inch spacing is used for 4–6 inch slabs and driveways. #5 rebar (5/8 inch) is used for structural slabs, foundations, and heavier loads.
What is rebar spacing for a concrete slab?
Standard residential slab rebar spacing is 12 to 18 inches on center in both directions. Driveways typically use 12-inch spacing. Patios may use 18-inch spacing. Structural slabs and footings are engineered to specific spacing requirements.
How much does rebar cost per linear foot?
#3 rebar typically costs $0.25–$0.40/linear ft. #4 runs $0.35–$0.55/ft. #5 runs $0.55–$0.80/ft. Prices fluctuate with steel commodity markets. Always get current quotes from your local supplier.
What is the difference between #3, #4, and #5 rebar?
Rebar is designated by eighths of an inch: #3 = 3/8 inch diameter (0.376 lb/ft), #4 = 1/2 inch diameter (0.668 lb/ft), #5 = 5/8 inch diameter (1.043 lb/ft). Higher numbers are heavier and stronger, used for structural applications.