Rebar sizes and nominal diameters
US standard rebar sizes are designated by numbers that indicate the bar's nominal diameter in eighths of an inch. A #3 rebar is 3/8 in (0.375 in, 0.376 lb/ft), a #4 rebar is 4/8 or 1/2 in (0.500 in, 0.668 lb/ft), and a #5 rebar is 5/8 in (0.625 in, 1.043 lb/ft).
For typical residential concrete flatwork—including 4-inch patios, walkways, garage floors, and driveways—#4 Grade 60 deformed rebar is the universal standard. Heavier commercial slabs or continuous trench footings typically step up to #5 rebar.
How do you calculate concrete cubic yards and bags?
Ready-mix concrete is ordered in cubic yards (1 yd³ = 27 ft³), while slab thickness is almost always measured in inches. Convert slab thickness to feet by dividing by 12, multiply by length and width to get cubic feet, and divide by 27:
where:
- L, W – Slab length and width in feet (find surface area first with the square footage calculator);
- V_cu yd – Exact geometric concrete volume in cubic yards (check haul weight with the concrete weight calculator); and
- Bags_80 – Estimated 80 lb premixed bags (⌈ V_cu ft ÷ 0.60 ⌉). To compare 40, 50, 60, and 80 lb bag sizes, open the concrete bag calculator.
| Bar Size | Nominal Diameter | Cross-Section Area | Weight per Linear Foot |
|---|---|---|---|
| #3 | 0.375 in (3/8") | 0.11 sq in | 0.376 lb/ft |
| #4 | 0.500 in (1/2") | 0.20 sq in | 0.668 lb/ft |
| #5 | 0.625 in (5/8") | 0.31 sq in | 1.043 lb/ft |
| #6 | 0.750 in (3/4") | 0.44 sq in | 1.502 lb/ft |
Grid spacing: 18-inch vs 12-inch on-center
Standard residential crack-control rebar grids are laid out at 18 inches on-center in both directions (a 18" × 18" square grid). This provides balanced tensile reinforcement throughout the middle third of the slab.
For areas carrying heavy vehicle loads, such as RV parking pads, heavy equipment sheds, or commercial aprons, engineers typically specify 12-inch on-center spacing or a double rebar mat to prevent punch-through shear and slab settlement.
Worked example: 20 ft by 20 ft driveway slab
Consider a 20 ft × 20 ft slab with 18-inch grid spacing, 3 inches of edge clearance, and standard #4 rebar. Deducting 3 inches on each side leaves a net reinforced span of 19.5 feet in both directions.
Dividing 19.5 feet by 1.5 feet (18 in) gives 13 spaces, requiring 14 lengthwise runs and 14 widthwise runs. Each run is 19.5 feet long (fitting within standard 20 ft stock bars). Total base footage is 28 × 19.5 = 546 linear feet. Adding a 10% cutting and trimming allowance yields 601 linear feet, which divides into thirty-one 20-foot stock bars weighing approximately 401 pounds.
| Component | Quantity / Measurement | Ordering Result |
|---|---|---|
| Net grid dimensions | 19.5 ft × 19.5 ft | 3 in concrete edge cover |
| Grid line count | 14 lengthwise + 14 widthwise | 28 total bar lines |
| Base linear footage | 28 lines × 19.5 ft | 546 linear feet |
| Footage with 10% allowance | 546 ft × 1.10 | 601 linear feet |
| Stock bars to purchase | 601 ft ÷ 20 ft stick | 31 bars (20 ft lengths) |
| Total steel weight | 601 ft × 0.668 lb/ft | 401.2 lbs (~0.20 tons) |
Lap splice requirements (ACI 318)
When a slab dimension exceeds standard stock bar lengths (e.g., a 30-foot or 40-foot run using 20-foot bars), bars must overlap at splices to transfer tension loads across the joint.
Under ACI 318 specifications, contact lap splices require an overlap of 30 to 40 bar diameters. For #4 bar (1/2 in diameter), 30 to 40 diameters equals 15 to 20 inches (18 inches is standard industry practice). Secure each lap splice with at least two steel wire ties.
Jobsite placement rules
- Use concrete dobies or plastic rebar chairs spaced every 2 to 3 feet to hold the rebar grid at the proper elevation (middle third or 1.5 inches from the bottom).
- Never lay rebar directly on subgrade dirt with the intention of pulling it up during the pour; wet concrete pushes steel right back to the ground.
- Maintain at least 3 inches of concrete cover when concrete is cast against and permanently exposed to earth.
- Tie at least 50% of rebar intersections with 16-gauge black annealed tie wire to keep the grid square during concrete pumping and foot traffic.