Rebar Calculator
Calculate rebar quantity, spacing, and weight for concrete reinforcement
Calculate Rebar
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How to Use This Calculator
Start by selecting your concrete project type: slab, foundation wall, column, or footing. Each application has different rebar requirements based on structural loads and building codes. Slabs typically use wire mesh or rebar grid patterns, foundation walls require vertical and horizontal rebar per engineering specs, and columns need vertical bars with ties. Understanding your project type helps the calculator apply appropriate spacing and sizing recommendations for code-compliant reinforcement.
Enter your project dimensions: length, width, and depth or height depending on project type. For a concrete slab, input length and width in feet to calculate the area requiring rebar reinforcement. The calculator determines the grid pattern needed—typically #3 or #4 rebar spaced 12-24 inches on center in both directions. For walls, input height and length to calculate vertical bar count and horizontal tie requirements. Accurate dimensions ensure proper material ordering and structural integrity.
Select rebar size from the dropdown menu: #3 (3/8″ diameter), #4 (1/2″), #5 (5/8″), #6 (3/4″), or larger. Most residential slabs use #3 or #4 rebar. Driveways, garage slabs, and commercial applications typically require #4 rebar for adequate strength. Foundation walls often use #4 or #5 vertical bars with #3 horizontal ties. Columns and beams may require #5, #6, or larger depending on load calculations. Check your engineering plans or local building code for required rebar sizes.
Input rebar spacing in both directions. Common spacing is 18 inches on center (O.C.) for residential slabs, 12-16 inches for heavier-duty applications like driveways or commercial floors, and 24 inches for light-duty patios. Closer spacing provides more strength but increases material cost. The calculator determines total linear feet of rebar needed, number of pieces required, total weight (important for delivery and handling), and estimated cost. Results include rebar tie wire needs, chairs or supports to elevate rebar to proper position within the concrete pour, and optional fiber reinforcement as an alternative or supplement.
Understanding Rebar Calculations
Rebar provides tensile strength to concrete, which is strong in compression but weak in tension. Concrete can support huge compressive loads (thousands of pounds per square inch) but cracks easily when stretched or bent. Rebar embedded in concrete handles tensile stresses, preventing cracking and catastrophic failure. For example, a driveway without rebar develops cracks from settling, heavy loads, and freeze-thaw cycles within 2-5 years. Properly reinforced driveways last 20-30+ years with minimal cracking.
Rebar sizing uses numerical designations indicating eighths of an inch diameter. #3 rebar is 3/8 inch diameter, #4 is 4/8 (1/2 inch), #5 is 5/8 inch, #6 is 6/8 (3/4 inch), and so on up to #18 (2-1/4 inches) for massive structural elements. Weight increases dramatically with size: #3 weighs 0.376 lbs per linear foot, #4 weighs 0.668 lbs/ft, #5 weighs 1.043 lbs/ft, and #6 weighs 1.502 lbs/ft. Material and delivery costs scale with weight, so using appropriate (not excessive) rebar size optimizes project economics.
Rebar spacing and grid patterns follow engineering principles and building codes. Typical residential slab reinforcement uses #3 or #4 rebar in 18-24 inch grid patterns, creating a net of steel bars running perpendicular to each other. Closer spacing (12-16 inches) provides more strength for heavy loads, expansive soils, or thicker slabs. Structural elements like foundation walls, columns, and beams require engineered rebar specifications with vertical bars, horizontal ties, and stirrups at specific spacing. DIYers can reinforce basic slabs following code minimums, but structural elements always require professional engineering.
Rebar must be properly positioned within the concrete pour to function effectively. In slabs, rebar sits 1/3 the depth from the bottom—for a 4-inch slab, rebar should be approximately 1.5 inches above the subgrade. Rebar chairs, bolsters, or dobies (supports) elevate rebar to correct height. During the concrete pour, walking on rebar can push it down to the bottom (ineffective) or to the top (also less effective). Use adequate supports every 3-4 feet and instruct concrete finishers to avoid standing on rebar. In walls, minimum cover is 1.5-3 inches depending on application—rebar too close to the surface causes rust staining and spalling.
Rebar tie wire and splicing affect structural integrity. Rebar intersections must be tied with wire (16-18 gauge) to prevent movement during concrete placement. Simply laying rebar in a grid without tying allows bars to shift out of position. Overlap splices are required when rebar isn't long enough—minimum lap is typically 40× bar diameter for #3-#6 bars (20 inches for #3, 30 inches for #4, 40 inches for #5, etc.). Proper splicing ensures continuous reinforcement without weak points. Some contractors use rebar couplers (mechanical connections) instead of lap splices in tight spaces, but these add $5-15 per connection.
Formula & Calculations
For slab reinforcement, calculate grid dimensions first. A 20×30 foot slab with 18-inch spacing: Length direction needs bars every 18 inches across 30-foot width. Convert to inches: 30 × 12 = 360 inches. Number of bars: 360 ÷ 18 = 20 bars, each 20 feet long. Total length: 20 bars × 20 feet = 400 linear feet. Width direction needs bars every 18 inches across 20-foot length: 20 × 12 = 240 inches ÷ 18 = 13.3, round to 14 bars, each 30 feet long. Total length: 14 × 30 = 420 linear feet. Combined total: 400 + 420 = 820 linear feet of rebar.
Convert linear feet to pieces, accounting for standard rebar lengths. Rebar is sold in 20-foot lengths (most common) or 40-foot lengths. Using 20-foot bars for our example: Length direction uses 20-foot bars directly = 20 pieces. Width direction needs 30-foot bars, requiring overlap of two 20-foot bars with 10-foot lap (conservative): 14 × 1.5 = 21 pieces of 20-foot bar. Total: 20 + 21 = 41 pieces of 20-foot #4 rebar. Add 10% waste: 41 × 1.10 = 45.1, round to 46 pieces.
Calculate total weight for delivery planning and cost estimation. #4 rebar weighs 0.668 lbs per foot. Our 46 pieces of 20-foot rebar = 920 linear feet × 0.668 lbs/ft = 614.6 lbs, or 0.31 tons. Rebar is sold by weight at approximately $0.50-0.90 per pound depending on market, or $600-800 per ton. Cost for this project: 615 lbs × $0.65/lb = $400 for rebar. Add accessories: rebar chairs ($0.40-0.75 each × ~75 needed) = $50, tie wire ($15 per roll, need 2 rolls) = $30. Total rebar reinforcement cost: $480.
For foundation walls, calculate vertical bar count and horizontal tie spacing. A 40-foot wall, 8 feet tall with vertical #4 bars every 16 inches: 40 feet × 12 = 480 inches ÷ 16 = 30 vertical bars. Each bar is 8 feet tall plus 12-18 inches embedment into footing plus any dowels above grade = 10-foot bars typically. Total: 30 bars × 10 feet = 300 linear feet. Horizontal ties are #3 bars wrapping around vertical bars every 16-18 inches vertically: 8 feet × 12 = 96 inches ÷ 16 = 6 horizontal levels. Each horizontal runs 40 feet perimeter: 6 × 40 = 240 linear feet of #3 rebar for ties. Total wall reinforcement: 300 linear feet #4 + 240 linear feet #3.
Key Factors to Consider
Rebar grade affects strength and cost significantly. Grade 40 rebar has 40,000 PSI yield strength, Grade 60 has 60,000 PSI. Grade 60 is standard for most residential and commercial projects—50% stronger than Grade 40 for the same size bar. This allows using smaller bars or wider spacing for equivalent strength. Grade 60 costs only 5-10% more than Grade 40, making it superior value. Some structural applications require Grade 75 (75,000 PSI) for maximum strength with minimum weight. Always specify Grade 60 minimum for new construction—most suppliers stock only Grade 60 anyway.
Epoxy-coated rebar costs 30-40% more than standard bare steel but prevents rust in harsh environments. Use epoxy-coated rebar for: coastal construction (salt air), bridge decks, parking structures with de-icing exposure, or any application where chlorides or moisture penetration is concern. Bare rebar rusts when moisture penetrates concrete, causing 2-4× volume expansion that spalls and cracks the concrete. In standard residential construction away from salt exposure, bare rebar is adequate and economical. In marine or industrial environments, epoxy coating is essential for long-term durability—repair costs from rust damage far exceed initial material premium.
Soil conditions dramatically affect rebar requirements and grid spacing. Expansive clay soils that swell when wet and shrink when dry create massive stresses in concrete slabs—these require closer rebar spacing (12-16 inches), heavier bars (#4 or #5), and often thickened slab edges or structural ribs. Sandy or well-draining soils are less demanding. Always obtain a geotechnical soil report ($500-1,500) for new construction—this identifies soil bearing capacity, expansiveness, and special foundation requirements. Building on unknown soils risks catastrophic foundation failure.
Rebar positioning within the concrete cross-section is critical. For slabs, rebar at mid-depth (1/2 way down) provides no benefit—concrete cracks from tension on the bottom. Proper position is lower 1/3 of depth: in a 6-inch slab, rebar should be 2 inches above the base. Use rebar chairs to maintain position—without supports, rebar sinks to the bottom during pouring (ineffective) or workers push it down while screeding. In beams, rebar sits near the bottom (tension zone). In columns, vertical rebar is placed in a circle or grid pattern near the outer perimeter with 1.5-2 inch concrete cover. Improper positioning reduces or eliminates rebar effectiveness.
Local building codes mandate minimum rebar requirements based on seismic zone, wind exposure, soil conditions, and application. Some jurisdictions require permits and inspections for concrete work—the inspector verifies proper rebar size, spacing, and positioning before allowing concrete pour. Failing inspection means removing all rebar and redoing the setup, or in worst cases, removing cured concrete. Always check local requirements before starting. In earthquake zones, rebar requirements are extensive with special seismic ties and hooks. Coastal high-wind areas require hurricane ties and enhanced reinforcement. Never assume "standard" rebar is adequate without verifying local codes.
Frequently Asked Questions
1Do I need rebar in a 4-inch concrete slab?
Yes, 4-inch residential slabs typically require reinforcement, though building codes vary. Most codes require either rebar grid or wire mesh (WWF) for driveways, garage floors, and basement slabs. Standard reinforcement is #3 or #4 rebar in 18-24 inch grid pattern, or 6×6 W1.4×W1.4 welded wire fabric. Slabs without reinforcement crack within 2-5 years from settling, shrinkage, and loads. Some exceptions exist: very small slabs (under 10×10 feet), non-structural patios in mild climates, or slabs with fiber-reinforced concrete may not require rebar. Check local building codes. The cost of rebar ($100-200 for typical residential slab) is cheap insurance against premature cracking and failure.
2What size rebar should I use for a driveway?
#4 rebar (1/2 inch diameter) in 18-inch grid pattern is standard for residential driveways. This provides adequate strength for passenger vehicles and light trucks. For driveways with RV parking, boat trailers, or frequent heavy delivery trucks, upgrade to #4 bars at 16-inch spacing or #5 bars at 18-inch spacing. Very light-duty driveways might use #3 rebar at 18-inch spacing, but the cost difference is minimal—#4 provides better long-term value. Position rebar in the lower 1/3 of slab thickness using rebar chairs (1.5-2 inches above base in a 4-inch slab). Add a thickened edge (6-8 inches deep) with additional rebar around the perimeter for extra support.
3How much does rebar cost?
Rebar costs $0.50-0.90 per pound or $600-900 per ton depending on size, grade, market conditions, and quantity. #3 rebar costs approximately $8-12 per 20-foot stick, #4 costs $13-18, #5 costs $20-28, and #6 costs $28-38. Epoxy-coated rebar costs 30-40% more. Prices fluctuate with steel markets. For a typical 500 sq ft garage slab (20×25 feet), #4 rebar in 18-inch grid requires approximately 850 linear feet or 43 pieces of 20-foot bar = 570 pounds × $0.65/lb = $370 for rebar plus $50-75 for chairs and wire = $420-450 total reinforcement cost. This represents about 15-20% of total concrete project cost.
4Can I use wire mesh instead of rebar?
Wire mesh (welded wire fabric or WWF) can replace rebar in some applications but is generally less effective. Standard 6×6 W1.4×W1.4 wire mesh is adequate for light-duty slabs like patios or walkways where structural loads are minimal. For driveways, garage floors, or basement slabs, #3 or #4 rebar in proper grid pattern provides superior crack control and strength. Wire mesh disadvantages: easily pushed to slab bottom during concrete placement (reducing effectiveness), breaks when walked on, and difficult to splice. Rebar advantages: stays in position with proper chairs, stronger, easier to work with, better for larger slabs. Use wire mesh only for non-critical, small-area applications. For all structural or load-bearing slabs, use rebar.
5How do you tie rebar together?
Tie rebar intersections using 16-18 gauge steel tie wire to prevent movement during concrete placement. Technique: cut 12-16 inch pieces of tie wire, wrap wire around both intersecting bars in a figure-8 pattern, twist tight with rebar tying pliers or tie wire tool. Each intersection needs one tie. For a 500 sq ft slab with 18-inch grid, expect 180-220 intersections requiring one 5-pound roll of tie wire ($15-20). Alternative: snap ties or pre-formed wire ties work faster but cost more. Don't skip tying—untied rebar shifts out of position during concrete pour. Some contractors use zip ties for temporary positioning, but code requires steel wire for permanent installation. Tie wire is also needed to secure rebar chairs and maintain proper spacing.
6Where should rebar be placed in a concrete slab?
Rebar belongs in the bottom third of a concrete slab to resist tensile stress from loads. For a 4-inch slab, position rebar approximately 1.5 inches above the subgrade using rebar chairs, bolsters, or dobies. Too low (on subgrade) provides no crack control. Too high (mid-depth or top third) is ineffective—slabs crack from tension on the bottom face. In 6-inch slabs, rebar sits 2 inches above base. Support spacing: use rebar chairs every 3-4 feet to prevent sagging. During concrete placement, prohibit workers from walking directly on rebar—use planks to distribute weight. In structural beams, rebar placement depends on load direction: bottom for positive moment (sagging), top for negative moment (hogging). Always verify placement with inspection before pouring.
7Do you put rebar in before or after pouring concrete?
Always install rebar BEFORE pouring concrete. Process: (1) Prepare subgrade with compacted gravel base, (2) Set forms to proper grade, (3) Place rebar on chairs at proper position (bottom 1/3 of slab depth), (4) Tie intersections with wire, (5) Verify spacing and elevation, (6) Call for inspection if required, (7) Pour concrete. Rebar must be fully embedded in concrete to provide reinforcement—this requires placement before the pour. During the pour, avoid standing on rebar or pushing it down. Some contractors place welded wire mesh after initial concrete placement and then pull it up to mid-depth—this technique works for wire mesh but not for rebar. Rebar cannot be moved after concrete hardens without compromising structural integrity.