Rebar Calculator

Calculate linear feet in each direction: (slab length ÷ spacing + 1) × slab width, then repeat for perpendicular direction. For 20×20 ft slab with 18" spacing: (20÷1.5+1)×20 + (20÷1.5+1)×20 = 560 linear feet. Divide by bar length for number of bars. Add 10-15% for overlaps and waste. Standard #4 rebar at 18" OC is common for residential slabs.

#3 (3/8") for light applications, temperature reinforcement. #4 (1/2") is standard for residential slabs, footings, walls. #5 (5/8") for heavier structural work. #6 (3/4") and larger for commercial/structural. Slab-on-grade typically uses #4 @ 18" OC. Foundations may require larger bars. Always follow engineering specifications or local building codes.

Rebar prices: USA $0.40-0.80/lb, $5-15 per 20ft bar; Canada C$8-18/bar; UK £8-20/bar; India ₹50-80/kg; EU €0.80-1.50/kg. Prices fluctuate with steel markets. Bulk orders reduce cost. Wire mesh may be more economical for slabs. Installation adds $0.50-1/lb. Total reinforcement typically 2-5% of concrete cost.

Standard splice/overlap is 40-50× bar diameter. #4 rebar: 40×0.5"=20" minimum overlap. #5: 25" overlap. #6: 30" overlap. Overlaps must be tied with wire. Stagger splices—don't align all overlaps at same location. In critical areas, engineers may specify longer laps or mechanical couplers. Add 10-15% to material estimate for overlaps.

Wire mesh (welded wire fabric) is faster to install and sufficient for residential driveways and patios—4" slabs typically use 6×6 W1.4/W1.4 mesh. Rebar provides more strength for thicker slabs, structural applications, and where point loads exist. Rebar is easier to keep properly positioned in concrete. Some contractors prefer rebar for better crack control. Check local code requirements.

Rebar should have minimum 3" (75mm) cover in ground contact, 1.5-2" (40-50mm) in formed work. Use rebar chairs, blocks, or wire supports to maintain position during pour. Place rebar in lower third of slab for maximum benefit. Tie intersections with wire. Keep rebar clean—dirt and oil reduce bond. Verify position after initial concrete placement.