Rebar Calculator

How Does the Formula Work?

The rebar calculator determines the quantity of reinforcing steel bars needed for a concrete slab, footing, or foundation. You enter the slab dimensions, the center-to-center bar spacing, and the bar size, and the tool computes the number of bars in each direction, the total linear footage, the weight of steel, and how many standard-length bars to purchase. A waste factor covers overlapping splices and cutting losses.

The "+1" in each bar count accounts for the first bar at the edge. For a 20-foot slab with 12-inch spacing, you get floor(20 / 1) + 1 = 21 bars across the width. The same logic applies in the perpendicular direction. Together these form a grid (mat) that distributes tensile forces evenly across the slab.

Rebar Sizes and Weight

In the US, rebar is designated by number, where each number represents 1/8 of an inch in diameter. #4 bar is 4/8 = 1/2 inch diameter and weighs 0.668 lb/ft (0.303 kg/ft). #5 is 5/8 inch at 1.043 lb/ft. Common residential choices are #3 and #4 for slabs, #5 for footings. At Home Depot, 1/2-inch (#4) 20-ft rebar runs about $8–12 per bar. Lowes carries 10-ft and 20-ft lengths. For large projects, buy from a steel supplier or rebar fabricator for better pricing; expect $0.50–0.80 per pound for bulk orders.

Spacing and Building Code

The IRC (International Residential Code) does not mandate reinforcement in a slab-on-ground. R506.1 lets the floor be built to that section or to ACI 332 and sets a minimum thickness of 3 1/2 inches; R506.3.4 says only that "where provided", reinforcement must be held between the middle and the upper third of the slab while the concrete is placed. No minimum bar size or spacing appears in R506. In practice #4 bar at 18 inches on center both ways is a common residential specification, and driveways and garage floors often use 12-inch spacing for added strength. Footings typically require #4 or #5 bars at closer spacing per the structural engineer's design. Always check your local building code and approved plans; this calculator gives quantities, not structural design advice.

Overlap and Waste

Rebar must overlap (splice) where bars meet end to end. There is no flat 40-diameter rule: ACI 318 sizes a tension lap as 1.0 times the development length for a Class A splice or 1.3 times it for Class B, and never less than 12 inches (25.5.2.1), so the length comes from your approved plans. These splices add material. Combined with cutting waste (pieces left over from cuts), a 10% waste factor is standard for simple rectangular slabs. For complex shapes, L-slabs, or many penetrations (plumbing, posts), use 15%. The calculator applies your waste percentage to the total length before computing weight and standard bar count.

Rebar Grid vs. Wire Mesh

For light-duty residential slabs, welded wire mesh (WWM) is an alternative to individual rebar. 6×6 W1.4/W1.4 mesh comes in rolls or sheets and is faster to install. However, rebar grids are structurally superior; they provide more steel per area and better crack control. Most structural engineers specify rebar for driveways, garage floors, and any slab thicker than 4 inches. This calculator is designed for rebar grids; for wire mesh, consult the manufacturer's coverage tables.

Rebar Sizes and Grades

Rebar in the US is designated by bar number which indicates diameter in eighths of an inch. A number 4 bar is 4/8 or 1/2 inch (12.7 mm) diameter. Number 3 (3/8 inch, 9.5 mm) is used for temperature and shrinkage reinforcement. Number 4 and 5 are standard for residential slabs, footings, and walls. Number 6 through 8 are used in commercial and structural applications. Number 9 through 11 are heavy structural bars. The most common grade is Grade 60 (60,000 PSI yield strength) designated ASTM A615. Grade 40 is used for light applications. Grade 75 and 80 are available for high-strength requirements. At Home Depot and local steel suppliers, number 4 rebar costs 5 to 10 dollars per 20-foot length. Number 5 costs 8 to 15 dollars.

Spacing and Placement

Rebar spacing depends on the structural design. For residential slab-on-grade, number 4 bars at 18 inches on center both ways is a common specification. For footings, number 4 or 5 bars are placed with 3 inches of clear cover from the bottom and sides. The calculator determines the number of bars needed based on slab dimensions and spacing; it calculates bars in both directions and accounts for overlap at splices. Lap splice length comes from the structural design; ACI 318 ties it to the development length rather than to a fixed multiple of the bar diameter. Add 10 percent waste for cutting and overlap.

Tying and Accessories

Rebar is tied at intersections using tie wire (16.5 gauge annealed wire) and a tie wire reel. You need approximately 3 to 5 ties per square foot of reinforcement grid. Bar chairs (supports) hold rebar at the correct height within the concrete; plastic chairs cost 10 to 20 cents each and are spaced every 3 to 4 feet. At Home Depot a 3.5-pound coil of tie wire costs about 5 dollars and ties approximately 200 intersections. For large projects consider a rebar tying tool from MAX or Makita which costs 300 to 600 dollars but increases speed by 5 times.

Cost Estimation

For a 20 by 20 foot slab with number 4 rebar at 12-inch spacing: the calculator determines 42 bars (21 each direction) at 20 feet each. That is 840 linear feet of rebar. Number 4 rebar weighs 0.668 pounds per foot so 840 feet weighs 561 pounds. At 0.50 to 0.75 dollars per foot materials cost 420 to 630 dollars. Add tie wire (15 dollars), bar chairs (30 dollars), and 10 percent waste. Total rebar materials for this slab: approximately 500 to 750 dollars. Professional rebar installation costs 0.50 to 1.00 dollar per square foot adding 200 to 400 dollars for labor.