Free Rebar Calculator
Estimate rebar bars, spacing, linear feet, weight, and optional cost for slabs, driveways, and foundations.
Last updated: March 31, 2026
Enter Your Measurements
Results
Rebar should be placed in the lower third of the slab for maximum tensile strength.
Project Examples
Driveway Slab
20 ft × 10 ft slab · #4 rebar · 18 in spacing
A small driveway slab usually lands around 15 standard 20 ft bars once you convert the grid into total linear feet.
Patio Slab
12 ft × 12 ft slab · #3 rebar · 18 in spacing
A patio layout stays lighter, but the order still rounds up fast because store stock lengths are usually sold in 20 ft bars.
Garage Floor
24 ft × 24 ft slab · #4 rebar · 16 in spacing
Larger garage floors push both bar count and weight quickly, so it helps to estimate stock lengths before calling a supplier.
How to Use This Calculator
- Select your project type - Choose from driveway, patio, foundation, or custom. This automatically sets recommended rebar size and spacing.
- Enter area dimensions - Measure your slab length and width in feet. For circular slabs, enter the diameter.
- Choose rebar size - #3 or #4 for residential slabs, #4 or #5 for driveways and foundations. The calculator shows weight per foot for each size.
- Set spacing - Standard spacing is 12-18 inches on center. Per ACI 318, maximum spacing is 3× slab thickness or 18", whichever is smaller.
- Select exposure condition - This determines the minimum concrete cover required to protect the rebar from corrosion.
How to Turn Slab Size Into a Rebar Order
Quick Example:
- • Slab: 20 ft × 10 ft = 200 sq ft
- • Rebar: #4 (0.668 lb/ft)
- • Spacing: 18" (1.5 ft) on center
- • Cover: 1.5" (outdoor exposure)
- • Length direction: 20 ÷ 1.5 + 1 = 14 bars
- • Width direction: 10 ÷ 1.5 + 1 = 8 bars
- • Total: 14 + 8 = 22 bars
- • Linear feet: (14 × 9.75) + (8 × 19.75) = 294.5 ft
- • Weight: 294.5 × 0.668 = 197 lbs
Rebar per Square Foot (Quick Reference)
- • 12\" spacing: ~2 linear ft/sq ft
- • 16\" spacing: ~1.5 linear ft/sq ft
- • 18\" spacing: ~1.33 linear ft/sq ft
- • 24\" spacing: ~1 linear ft/sq ft
Spacing Guidelines for Concrete Slabs
- Residential slabs: 18-24 inches center to center (both directions).
- Expansive soil: Use 16-inch spacing for better crack control.
- ACI 318 max: 3× slab thickness or 18\", whichever is smaller.
Rebar Cost Guide
Rebar typically costs $0.75-$1.40 per linear foot depending on size (#3, #4, #5) and location. Add 10-15% for overlaps. Use our calculator above to get total linear feet, then multiply by your local price.
Rebar spacing and cover requirements per ACI 318-19 standards. Rebar specifications per ASTM A615. Learn more about ACI standards