Rebar Calculator

Calculate rebar quantities, spacing, and weight for concrete slabs, driveways, and foundations. Get instant reinforcement estimates.

Slab dimensions

Enter measurements for concrete areas requiring rebar

Standard: 24" for #4, 30" for #5, 36" for #6

$

Typical: $12-18 for 20ft #4 bar

Complete Guide to Calculating Rebar for Concrete Reinforcement

Proper rebar calculation ensures structural integrity while controlling costs in concrete projects. Whether you're pouring a driveway, patio, garage floor, or foundation, accurate rebar quantities prevent under-reinforcement that compromises strength or over-ordering that wastes money. Understanding rebar sizing, spacing, and placement requirements helps you build code-compliant structures that withstand loads and resist cracking for decades.

Rebar calculation starts with slab dimensions and spacing requirements. Measure the length and width of your concrete area in feet, select appropriate rebar spacing (typically 18 inches on center for residential applications), then calculate how many bars run in each direction. Divide width by spacing to determine bars running lengthwise, divide length by spacing for bars running widthwise, add totals together, and multiply each direction by slab dimensions for total linear feet. For a 10×20 foot driveway with standard 18-inch spacing, you need approximately 14 bars at 20 feet each plus 7 bars at 10 feet each, totaling 350 linear feet or 18 standard 20-foot bars.

Choosing the Right Rebar Size for Your Project

Rebar size selection depends on application, load requirements, and building codes. Rebar is designated by numbers indicating diameter in eighths of an inch—#4 rebar measures 4/8 or 1/2 inch diameter. Common sizes include #3 rebar (3/8 inch diameter, 0.376 lbs per foot) for light residential applications like small patios and walkways under 4 inches thick, #4 rebar (1/2 inch diameter, 0.668 lbs per foot) as the standard for residential slabs, driveways, and garage floors 4-6 inches thick, and #5 rebar (5/8 inch diameter, 1.043 lbs per foot) for heavy-duty driveways, commercial slabs, and foundations supporting significant loads.

Larger sizes like #6 (3/4 inch, 1.502 lbs/ft), #7 (7/8 inch, 2.044 lbs/ft), and #8 (1 inch, 2.67 lbs/ft) are used for structural beams, columns, and heavy commercial applications. Most DIY residential projects use #4 rebar as it provides adequate strength for typical loads while remaining affordable and easy to work with. Building codes specify minimum rebar sizes and quantities for different applications— always verify local requirements before starting construction. Using undersized rebar risks structural failure, while oversizing increases costs without proportional benefit.

Understanding Rebar Spacing and Grid Patterns

Proper rebar spacing creates a reinforcement grid that distributes loads throughout the concrete slab. Standard residential spacing is 18 inches on center in both directions, meaning bars are placed 18 inches apart measured from center to center. This grid pattern works for most driveways, garage floors, and patios. Tighter 12-inch spacing provides extra reinforcement for heavy traffic areas, high-load zones, thin slabs needing additional support, and areas with poor soil conditions or high water tables.

Wider 24-inch spacing suffices for light-duty applications like shed floors, covered patios, and walkways with minimal loads. Rebar must be positioned correctly within the slab depth—place bars in the lower third for maximum effectiveness against tensile stress. For a 4-inch thick slab, position rebar approximately 1.5 inches from the bottom using plastic or metal rebar chairs spaced every 3-4 feet. Maintain 3 inches minimum distance from all slab edges to prevent exposure and corrosion. Never place rebar directly on dirt—concrete must fully encapsulate the steel for corrosion protection and proper bonding.

Calculating Overlap Length and Splicing Requirements

Rebar overlaps transfer stress between bars when standard lengths don't span entire dimensions. Standard overlap length is 40 times the bar diameter, meaning #4 rebar (0.5 inch diameter) requires 20-inch minimum overlap, though 24 inches is standard practice providing extra safety margin. Larger bars need proportionally longer overlaps: #5 rebar requires 30-inch overlap, #6 needs 36-inch overlap, and #7 requires 42-inch overlap.

All overlaps must be tied together with wire at multiple points— typically three ties per overlap zone ensure bars stay positioned during concrete placement. When calculating total rebar needed, add extra length for overlaps by estimating that 20% of total bars will require splicing. Never overlap rebar at high-stress locations like corners, mid-span under heavy loads, or near large openings. Stagger overlap locations so multiple splices don't align in the same area, which would create weak zones. For critical applications, consult structural engineers regarding splice locations and overlap lengths as building codes may require specific configurations.

Weight Calculations and Transportation Planning

Rebar weight matters significantly for transportation, handling, and cost calculations. Weight per linear foot increases dramatically with bar size: #3 weighs 0.376 lbs/ft, #4 weighs 0.668 lbs/ft, #5 weighs 1.043 lbs/ft, #6 weighs 1.502 lbs/ft, #7 weighs 2.044 lbs/ft, and #8 weighs 2.67 lbs/ft. A typical 20-foot #4 bar weighs approximately 13.4 pounds. A bundle of 40 bars weighs roughly 535 pounds, while larger bundles of 80 bars exceed half a ton.

Always verify vehicle payload capacity before transporting rebar— standard pickup trucks handle 1,500-2,000 pounds safely while trailers vary by rating. Many suppliers offer delivery service, charging by weight and distance. For our example 10×20 driveway requiring 18 bars of #4 rebar, total weight equals approximately 240 pounds, easily transportable in most vehicles. Larger projects requiring hundreds of bars necessitate professional delivery. Factor delivery costs into project budgets or arrange appropriate transportation. Rebar handling requires caution as steel bars are heavy, unwieldy, and can cause injury—use proper lifting techniques and wear work gloves.

Frequently Asked Questions

How do I calculate how much rebar I need for a concrete slab?
Calculate rebar quantity by determining slab dimensions in feet, choosing rebar spacing (typically 18 inches on center for driveways and patios), dividing width by spacing to get number of bars running lengthwise, dividing length by spacing to get number of bars running widthwise, adding both totals together, then multiplying by slab length or width for total linear feet. For a 10×20 foot slab with 18-inch spacing: width 10ft ÷ 1.5ft spacing = 7 bars lengthwise (20ft each = 140 linear feet), length 20ft ÷ 1.5ft spacing = 14 bars widthwise (10ft each = 140 linear feet), totaling 280 linear feet. Divide by standard bar length (20 feet) to get 14 bars needed.
What rebar size should I use for my concrete project?
Rebar size depends on application and load requirements. Use #3 rebar (3/8 inch diameter) for light residential applications like small patios and walkways under 4 inches thick. Use #4 rebar (1/2 inch diameter) for standard residential slabs, driveways, and garage floors 4-6 inches thick—most common size for DIY projects. Use #5 rebar (5/8 inch diameter) for heavy-duty driveways, commercial slabs, and foundations supporting significant loads. Use #6 and larger for structural beams, columns, and heavy commercial applications. Building codes specify minimum rebar sizes for different applications—always check local requirements.
What is the standard spacing for rebar in concrete slabs?
Standard rebar spacing for residential slabs is 18 inches on center in both directions, creating a grid pattern. This spacing works for most driveways, garage floors, and patios. Some applications require tighter spacing: 12-inch spacing for heavy traffic areas, high-load zones, or thin slabs needing extra reinforcement, while light-duty applications like shed floors may use 24-inch spacing. Rebar should be placed in the lower third of the slab depth using chairs or supports—for a 4-inch slab, position rebar approximately 1.5 inches from the bottom. Always verify spacing requirements with local building codes and structural engineers for critical applications.
Do I need to overlap rebar when bars meet?
Yes, rebar must overlap at splice points to transfer stress between bars. Standard overlap length is 24 inches (40 times the bar diameter for #4 rebar) for most residential applications. Larger bars require longer overlaps: #5 rebar needs 30-inch overlap, #6 needs 36-inch overlap, and #7 needs 42-inch overlap. Overlaps must be tied together with wire at multiple points. When calculating rebar quantities, add extra length for overlaps—estimate 20% of total bars will need splicing. Never overlap rebar at high-stress areas like corners or mid-span under heavy loads.
How much does rebar weigh and why does it matter?
Rebar weight varies by size and is measured in pounds per linear foot. Common sizes weigh: #3 rebar weighs 0.376 lbs/ft, #4 weighs 0.668 lbs/ft, #5 weighs 1.043 lbs/ft, #6 weighs 1.502 lbs/ft, #7 weighs 2.044 lbs/ft, and #8 weighs 2.67 lbs/ft. Weight matters for transportation planning (vehicle payload limits), delivery costs (charged by weight), handling safety (heavier rebar requires equipment), and structural calculations (dead load on foundations). A bundle of 40 pieces of 20-foot #4 rebar weighs approximately 535 pounds. Always verify your vehicle or trailer can handle the load before transporting rebar.
Can I use wire mesh instead of rebar?
Wire mesh (welded wire reinforcement) works for light-duty applications but differs from rebar significantly. Wire mesh is suitable for sidewalks, thin slabs under 4 inches, light patios, and crack control in non-structural slabs. Rebar is required for driveways supporting vehicles, structural slabs supporting buildings, slabs over 4 inches thick, and areas with heavy loads. Wire mesh comes in rolls and is easier to install but provides less reinforcement. Common mesh sizes are 6×6 inch (W1.4×W1.4) for light duty and 4×4 inch (W2.9×W2.9) for moderate duty. Building codes specify when rebar is mandatory—mesh cannot substitute for code-required rebar.
How far from the slab edge should rebar be placed?
Rebar must maintain proper edge distance and depth for effectiveness and corrosion protection. Position rebar 3 inches minimum from all slab edges to prevent exposure and corrosion. For slab depth, place rebar in the lower third—in a 4-inch slab, position rebar 1.5 inches from bottom using chairs or supports. Never place rebar on bare ground; concrete must fully encapsulate the steel. For slabs on grade, use plastic or metal rebar chairs spaced every 3-4 feet. Edge distance is critical at expansion joints and control joints—maintain 2-3 inches from all intentional slab separations.
How much does rebar cost per bar?
Rebar costs vary by size, length, and market conditions. Typical prices for 20-foot lengths: #3 rebar costs $8-12 per bar, #4 rebar costs $12-18 per bar (most common size), #5 rebar costs $18-28 per bar, #6 rebar costs $28-42 per bar, and larger sizes increase proportionally. Epoxy-coated rebar costs 30-50% more than standard black steel but prevents corrosion in harsh environments. Prices fluctuate with steel markets—check current local pricing. Buying in bulk (bundles of 40-50 bars) saves 10-20% compared to individual bars. Factor in delivery fees for large orders as rebar is heavy and difficult to transport.

© 2026 Tool Vault - Free rebar calculator for concrete reinforcement