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Rebar calculator diagram showing rebar grid spacing in a concrete slab 20 feet 10 feet 12 inch spacing 12 inch spacing
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Results

Total Rebar Bars
32
bars
20 ft × 10 ft slab · #4 rebar at 12 in spacing

Bar Count

Bars Along Length11 bars
Bars Along Width21 bars
Total Bars32 bars

Length & Weight

Total Length430 ft
Pieces Needed (20 ft stock)24 × 20 ft bars
Weight per Foot0.668 lbs/ft
Total Weight287 lbs
Weight (with waste)316 lbs

Cost Estimate

Rebar Cost$172.00
Total Cost (with waste)$189.20

Calculation Assumptions

Slab Size20 ft × 10 ft
Rebar Size#4 (1/2 inch)
Spacing12 in
Weight/ft0.668 lbs/ft
Waste Allowance10%
Edge Inset0 — bars run to slab edge

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Written & Reviewed by Editorial Team.

MyBuilderBase is an independent construction calculator and information project. Content is researched and prepared with the assistance of AI tools, followed by editorial review for clarity, consistency, calculation logic, and usability.

Editorial review is not professional engineering, architectural, or building-code approval.

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Last updated: September 12, 2026

What Is Rebar

Rebar (short for reinforcing bar) is a steel rod used to reinforce concrete structures. Concrete is strong in compression but weak in tension — rebar provides the tensile strength that concrete lacks, preventing cracking and structural failure.

Rebar comes in various sizes (numbered #3 through #18) and is placed in a grid pattern within concrete slabs, footings, walls, and columns. The bars are tied together at intersections with wire to maintain proper spacing during the concrete pour.

Steel rebar grid laid out in a residential driveway form before pouring

How to Calculate Rebar

Calculating rebar for a rectangular concrete slab follows a straightforward process:

  • Step 1: Measure the slab — Record the length and width of the concrete slab in feet.
  • Step 2: Choose rebar size and spacing — Select the appropriate rebar size based on your project type and load requirements. Common spacing is 12 to 18 inches on center.
  • Step 3: Calculate bars in each direction — Divide the slab width by the spacing and add 1 to get bars running along the length. Divide the slab length by the spacing and add 1 to get bars running along the width.
  • Step 4: Calculate total length — Multiply the number of bars in each direction by the perpendicular dimension to get total linear feet of rebar needed.
  • Step 5: Calculate weight and cost — Multiply total length by the weight per foot for your rebar size. Add 5–10% for waste and cuts.

Formula: Bars = (Dimension ÷ Spacing) + 1

Rebar Sizes & Weights

Rebar is sized by number, where the number represents the diameter in eighths of an inch. The most common sizes for residential and commercial construction are #3 through #8, which the calculator above covers. Larger sizes appear in heavy commercial, industrial, and civil work.

Size Diameter Weight (lbs/ft) Common Use
#33/8 inch0.376Light slabs, patios
#41/2 inch0.668Driveways, standard slabs
#55/8 inch1.043Heavy slabs, foundations
#63/4 inch1.502Commercial slabs
#77/8 inch2.044Heavy structures
#81 inch2.670Structural columns
#91.128 inch3.400Heavy commercial columns, walls
#101.270 inch4.303Industrial slabs, retaining walls
#111.410 inch5.313Piers, heavy foundations
#141.693 inch7.650Large columns, mat foundations
#182.257 inch13.600Bridges, heavy civil structures

Rebar Spacing Guide

The spacing of rebar depends on the slab thickness, load requirements, and local building codes. Here are common spacing recommendations:

Rebar spacing guidelines infographic showing different spacing for various applications
Application Recommended Spacing
Light-duty patio (4 inch)18 inches on center
Standard driveway (4–6 inch)12 inches on center
Heavy-duty slab (6+ inch)10 inches on center
Commercial/industrial8–12 inches on center

Note: "On center" means the distance is measured from the center of one bar to the center of the next. Always consult local building codes for specific requirements.

Tips for Installing Rebar

  • Maintain proper concrete cover — Rebar should be at least 3 inches from the edge of the slab and 2 inches from the bottom. Use chairs or stakes to hold rebar at the correct height.
  • Tie intersections securely — Use rebar wire ties at every intersection to keep the grid stable during the concrete pour. A simple figure-8 tie works well for most applications.
  • Overlap bars at joints — When bars are shorter than the slab dimension, overlap them by at least 40 bar diameters (e.g., 20 inches for #5 rebar) to maintain continuity.
  • Check for level — Ensure the rebar grid is level and at the correct height before pouring. Uneven rebar leads to inconsistent concrete cover and potential corrosion.

Material Costs

Rebar costs vary by size, length, and region. Here are typical 2026 price ranges:

Rebar Size Price per Linear Foot
#3 (3/8 inch)$0.20 – $0.40
#4 (1/2 inch)$0.30 – $0.60
#5 (5/8 inch)$0.50 – $0.90
#6 (3/4 inch)$0.70 – $1.20
#7 (7/8 inch)$0.90 – $1.50
#8 (1 inch)$1.10 – $1.80

Worked Example

Let us calculate rebar for a 20-foot × 10-foot driveway slab using #4 rebar at 12-inch spacing with 10% waste allowance.

Step 1: Bars along the length

10 ft ÷ 1 ft = 10, + 1 = 11 bars (running the full 20 ft length)

Step 2: Bars along the width

20 ft ÷ 1 ft = 20, + 1 = 21 bars (running the full 10 ft width)

Step 3: Total bars

11 + 21 = 32 bars

Step 4: Total length

(11 × 20 ft) + (21 × 10 ft) = 220 + 210 = 430 ft

Step 5: Total weight

430 ft × 0.668 lbs/ft = 287 lbs

Step 6: Add 10% waste

287 lbs × 1.10 = 316 lbs

Step 7: Pieces to order (20 ft stock)

430 ft × 1.10 = 473 ft with waste ÷ 20 ft = 23.65 → round up = 24 pieces

Frequently Asked Questions

How do I calculate how much rebar I need? ▼
Measure your slab length and width, choose a rebar size and spacing, then divide each dimension by the spacing and add 1 to get the number of bars in each direction. Add both quantities for the total number of bars. Multiply by the bar length and weight per foot to get total weight.
What size rebar do I need for a concrete slab? ▼
For standard residential slabs (patios, walkways), #3 or #4 rebar is common. Driveways and heavier loads typically use #4 or #5. Commercial and industrial projects may require #5 or larger. Always consult local building codes.
What is the standard spacing for rebar in a slab? ▼
Rebar is commonly spaced 12 to 18 inches on center for residential slabs. Heavier loads may require 10 to 12 inch spacing. The most common spacing for driveways is 12 inches on center.
How much does #4 rebar weigh per foot? ▼
#4 rebar (1/2 inch diameter) weighs 0.668 pounds per linear foot. A standard 20-foot piece weighs about 13.4 pounds.
How far should rebar be from the edge of concrete? ▼
Rebar should be placed with at least 3 inches of concrete cover from the edge and 2 inches from the bottom of the slab. This protects the steel from corrosion and ensures proper bonding with the concrete.
How many 20-foot rebar pieces do I need? ▼
Divide your total bar length, including the waste allowance, by the stock bar length and round up. For a 20 ft × 10 ft slab with 12-inch spacing: 430 ft × 1.10 = 473 ft with waste, and 473 ÷ 20 = 23.65, so order 24 twenty-foot pieces. Every cut leaves an offcut, which is why the waste allowance matters when counting full bars.
Do I need rebar for a 4-inch concrete slab? ▼
Rebar is recommended for most concrete slabs, including 4-inch slabs. It helps prevent cracking and adds structural strength. For light-duty slabs like small patios, wire mesh may be sufficient, but rebar provides better reinforcement.

References

  1. American Concrete Institute, "Building Code Requirements for Structural Concrete," ACI 318-19, concrete.org
  2. Concrete Reinforcing Steel Institute, "CRSI Design Handbook," crsi.org
  3. ASTM International, "Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement," ASTM A615, astm.org
  4. International Code Council, "International Building Code (IBC)," Chapter 19 — Concrete, iccsafe.org

Disclaimer: The rebar calculator results on this page are planning estimates only. Actual bar counts and lengths vary with slab geometry, lap splice requirements, and chair spacing, and local codes may mandate sizes or spacing this tool does not model. Confirm bar size, grade, and pricing with your supplier, and have all structural reinforcement designed or reviewed by a qualified engineer.

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