Rebar Calculator
Calculate rebar quantity for concrete slabs and footings — enter dimensions, spacing, and bar size to find total bars, length, and weight.
Slab Dimensions
Enter your slab dimensions and click Calculate.
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.
Bars along width = floor(length / spacing) + 1
Total length = (bars_L × L) + (bars_W × W)
Weight = total length × weight per unit length
Standard bars = ceil(total length / standard bar length)
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 (0.271 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) specifies minimum reinforcement for residential concrete. A standard 4-inch slab typically requires #4 bar at 18 inches on center (OC) both ways. 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. Standard splice length is 40 bar diameters — for #4 bar, that is 40 × 0.5 = 20 inches. 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. Standard splice length is 40 times the bar diameter: a number 4 bar requires a 20-inch lap splice. 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 40 bars (20 each direction) at 20 feet each. That is 800 linear feet of rebar. Number 4 rebar weighs 0.668 pounds per foot so 800 feet weighs 534 pounds. At 0.50 to 0.75 dollars per foot materials cost 400 to 600 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.
Tips & Recommendations
Always run bars in both length and width to create a grid. Single-direction rebar only resists cracking in one axis.
Where bars overlap, lap them by 40× the bar diameter minimum. For #4 bar: 40 × 0.5 in = 20 inches of overlap.
Rebar must sit in the middle or lower third of the slab, not on the ground. Use bar chairs (supports) every 3–4 feet.
Overlaps and cuts always consume more than expected. A 10% waste factor is the minimum — use 15% for complex shapes.
Frequently Asked Questions
How much rebar for a 20×20 ft slab?
With #4 bar at 12-inch spacing: 21 bars each direction = 42 bars, 840 ft total. At 0.271 kg/ft ≈ 228 kg (502 lb). With 10% waste: 924 ft, 250 kg. Buy 47 standard 20-ft bars.
What rebar spacing for a residential slab?
Most residential slabs use 12–18 inch spacing (on center). 12 inches is standard for driveways and garages. 18 inches for light-duty patios. Check your local building code — the IRC typically requires #4 bar at 18 in OC for a 4-inch slab.
What size rebar for a concrete slab?
#3 (3/8 in) for thin slabs and patios. #4 (1/2 in) for standard residential slabs, driveways, and sidewalks. #5 (5/8 in) for heavy-duty slabs and footings. #6 and above for structural and commercial applications.
Do I need rebar in both directions?
Yes. A rebar grid (mat) with bars running both ways prevents cracking from loads in any direction. This calculator automatically computes bars in both the length and width directions.
How many 20-ft bars should I buy?
The calculator shows the number of standard bars needed. Always round up and add 10% waste for overlaps and cuts. Rebar at Home Depot and Lowes comes in 10-ft and 20-ft lengths.
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