Rebar Calculator

How to Calculate Rebar for a Concrete Slab?

Rebar in a slab is laid in a grid pattern running in both directions. Count the number of bars in each direction: divide the slab dimension by the grid spacing and add one. For a 20 x 20-foot slab at 16-inch spacing: 20 feet = 240 inches / 16 = 15, plus 1 = 16 bars in each direction, for a total of 32 bars. Each bar spans the full slab dimension. The calculator above factors in the bar size, spacing, available rebar lengths, overlap requirements, and provides total bar count, linear footage, weight, and tie wire count.

What Rebar Sizes Are Used in Residential Construction?

Rebar is designated by number, where the number equals the diameter in eighths of an inch. #3 rebar is 3/8-inch diameter, weighing 0.376 pounds per foot. #4 is 1/2-inch at 0.668 lbs/ft and is the most common size for residential slabs, footings, and walls. #5 is 5/8-inch at 1.043 lbs/ft, used for heavier structural applications. #6 is 3/4-inch at 1.502 lbs/ft, used in commercial foundations and retaining walls. Residential slabs and footings almost always use #4 rebar. Walls and columns may use #4 or #5 depending on the structural engineer specifications and the loads involved.

What Grid Spacing Should I Use?

Common rebar grid spacings are 12, 16, 18, and 24 inches on center. The spacing depends on the concrete application and structural requirements. Residential driveways and garage slabs typically use #4 rebar at 18-inch spacing in both directions. Footings use #4 at 12-inch spacing or as specified by the structural engineer. Walls use vertical bars at 48-inch spacing (standard residential) or 16-24 inches for structural walls. Closer spacing provides more reinforcement per square foot but uses more material and labor. The calculator adjusts bar count and total weight based on your selected spacing.

How Much Rebar Overlap Is Required?

When rebar lengths are shorter than the slab dimension, bars must overlap at splices. The IRC requires a minimum overlap of 40 bar diameters. For #4 rebar (1/2-inch diameter): 40 x 0.5 = 20 inches. For #5 rebar: 40 x 0.625 = 25 inches. For #3: 40 x 0.375 = 15 inches. These overlaps are wired together with tie wire at every intersection. Standard rebar comes in 20-foot lengths. A 30-foot slab dimension requires two bars with a 20-inch overlap, consuming a total of about 31.7 feet of rebar per line. The calculator accounts for overlaps based on your selected bar length and slab dimensions.

How to Position Rebar in a Slab?

Rebar should sit in the lower third of the slab thickness for standard ground-bearing slabs. For a 4-inch slab, the rebar should be approximately 1.5-2 inches above the base (on 2-inch bar chairs). For elevated slabs (supported by beams), rebar placement follows the structural engineer drawings and may include both top and bottom layers. Plastic or wire bar chairs (rebar supports) hold the rebar at the correct height during the concrete pour. Without chairs, the rebar sinks to the bottom of the slab where it provides no structural benefit. Tie the rebar grid to the chairs at regular intervals to keep everything in position as workers pour and vibrate the concrete.

How Many Tie Wires Do I Need?

Tie wire secures rebar at grid intersections and splice points. You do not need to tie every single intersection. Tie every other intersection in a checkerboard pattern for slabs. Tie every intersection for walls and columns where bar displacement during the pour is more likely. A 20x20-foot slab at 16-inch spacing has approximately 256 intersections (16 bars x 16 bars). Tying half requires about 128 ties. Each tie uses approximately 12 inches of tie wire. A 3.5-pound roll of 16-gauge tie wire contains roughly 350 feet, enough for approximately 350 ties. One roll covers most residential slab projects.

Rebar vs Welded Wire Mesh

Welded wire mesh (WWF/WWR) is a pre-fabricated grid of welded steel wires available in flat sheets or rolls. The common specification 6x6-W2.9xW2.9 has 6-inch grid spacing with wires equivalent to about 10 gauge. Wire mesh is faster to install than individual rebar and is adequate for standard residential sidewalks, patios, and lightly loaded slabs. Rebar provides stronger reinforcement, handles heavier loads, and is required for structural applications like footings, foundation walls, driveways supporting heavy vehicles, and any engineer-specified work. Many builders use rebar for driveways and garage slabs even when mesh would technically suffice, because rebar stays in position on chairs while mesh tends to drop to the bottom of the slab during the pour.

How Much Does Rebar Cost?

Standard #4 rebar costs $0.50-$1.00 per linear foot at building supply retailers. A 20-foot bar runs $10-$20. #3 rebar is slightly cheaper at $0.35-$0.75 per foot. #5 costs $0.75-$1.50 per foot. Prices fluctuate with steel markets. For a 20x20-foot slab at 16-inch spacing, you need approximately 640 linear feet of #4 rebar, costing $320-$640. Add $10-$20 for tie wire and $20-$40 for bar chairs. The rebar material cost for a residential slab is typically 5-10% of the total concrete project cost. Cutting rebar on-site requires a rebar cutter or angle grinder with a metal cutoff wheel.

Frequently asked questions