Calculate rebar quantity, spacing, lap splices, and total weight for concrete slabs, footings, and walls.
Enter your concrete dimensions and rebar specifications
Standard: 18" for #4 (30× bar diameter). 0 = no splices (bars span full length)
Select your project type — slab (flat grid), footing (linear runs), or wall (vertical grid). Enter the dimensions for your project. Choose rebar size (#3 through #6) and spacing (12", 18", or 24" on center). Set the lap splice length — standard is 30× bar diameter (e.g., 18" for #4). The calculator shows pieces needed, total linear feet, and weight in both pounds and tons for ordering.
Concrete is strong in compression but weak in tension — roughly 10% of its compressive strength. Rebar (reinforcing steel bar) handles the tension loads, turning brittle concrete into a ductile composite material that can span openings, resist cracking, and survive freeze-thaw cycles. Getting the rebar layout right is the difference between a slab that lasts 50 years and one that cracks in the first season.
Rebar is numbered in eighths of an inch — #4 = 4/8" = 1/2" diameter. The grade (stamped on the bar) tells you the yield strength: Grade 40 (40,000 PSI) is older stock; Grade 60 (60,000 PSI) is the modern standard for almost all residential and commercial work; Grade 75 and 80 are used in high-rise and seismic zones. For residential projects, always specify Grade 60 — it's what every supplier stocks.
| Bar Size | Diameter | Weight (lbs/ft) | Typical Use |
|---|---|---|---|
| #3 | 3/8" | 0.376 | Patio slabs, sidewalks, stair treads |
| #4 | 1/2" | 0.668 | Standard slabs, driveways, residential footings |
| #5 | 5/8" | 1.043 | Retaining walls, foundation walls, heavy footings |
| #6 | 3/4" | 1.502 | Grade beams, bridge decks, structural columns |
Rebar spacing is a balance: too close together wastes steel and makes concrete placement difficult; too far apart and the slab can crack between bars. The rules of thumb:
Rebar must sit at the right depth within the concrete to work. Too close to the top or bottom and it loses effectiveness; too close to the surface and it rusts. The standard is 2 inches of concrete cover from all edges for slabs cast against the ground, and 1.5 inches for formed concrete exposed to weather. This cover distance is critical — it's what protects the steel from moisture and corrosion. Place rebar on concrete chairs or dobies (not bricks, which wick moisture into the slab). For a 4-inch slab, use 2-inch chairs — this centers the bar vertically. Never let rebar sit directly on the subgrade or be pulled up during the pour (common mistake: workers hook the rebar with a rake and lift it "close enough" — it isn't).
Rebar comes in standard lengths — 20 feet for most sizes, sometimes 40 or 60 feet on special order. When your slab is longer than the bar, you need a lap splice: overlapping two bars so the concrete transfers load from one to the other through bond stress. The standard lap length is 30× the bar diameter for Grade 60 rebar in normal-weight concrete (per ACI 318). For #4 bar (1/2"): 30 × 0.5 = 15 inches minimum, rounded up to 18" for a safety margin. For #5: 30 × 0.625 = 18.75", use 20-24". Lap splices should be staggered — don't align all splices in one row or you create a weak plane. Offset adjacent bar splices by at least 12 inches.
Continuous concrete footings (the strip footings under foundation walls) use a different rebar layout than slabs. For a typical residential footing (18" wide × 8" deep), place two #4 bars running longitudinally (lengthwise) — one in the top third and one in the bottom third of the footing depth. If the footing is wider than 24", add a third bar. Vertical dowels (#4 at 24" OC) tie the footing to the foundation wall above. For footings in seismic zones, code may require #4 stirrups (U-shaped ties) at 24" OC wrapping around the longitudinal bars.
Rebar suppliers think in three units: pieces (quantity of a specific length), linear feet (total length regardless of piece count), and tons (for pricing and delivery). A typical order: "40 pieces of #4 Grade 60 rebar, 20-foot lengths, with 90-degree bends on both ends, 6-inch legs." Standard bends add $1-3 per bend. Delivery fees run $50-150 depending on distance. Most suppliers will cut to length for $1-2 per cut. If your project is under 500 lbs total, it's usually cheaper to buy from a home center than a steel supplier (who may have a minimum order).