Rebar Calculator: Calculate Linear Feet for Concrete Reinforcement

You're pouring a concrete slab and you need to know: how much rebar do I need for reinforcement, and what spacing is code-compliant?

Rebar (reinforcing steel bar) strengthens concrete, preventing cracking under stress. It's laid in a grid pattern-typically 12 or 18 inches on center (spacing between bars). Calculating rebar needs depends on slab size, bar spacing, and bar size (#4 and #5 are most common residential). Get this right and your concrete lasts decades; cut corners and it cracks prematurely. Our rebar calculator figures out exactly what you need.

What This Calculator Does

This tool calculates rebar required for a concrete slab based on length, width, bar size, and grid spacing. You input the slab dimensions in feet, select rebar size (#4 or #5, most common), specify grid spacing (12 or 18 inches on center), and the calculator estimates total linear feet of rebar needed and how many bars to order. It accounts for overlap at joints (standard 24-40 bar diameters) and includes a 5% waste factor.

How to Use This Calculator

Step 1: Measure your slab length and width in feet. For irregular slabs, measure the largest dimensions or break it into sections.

Step 2: Choose rebar size. #4 (1/2-inch diameter) is standard for residential driveways and patios. #5 (5/8-inch) is used for heavier loads or thicker slabs. Larger sizes (#6, #7) are for industrial work.

Step 3: Decide grid spacing. 12 inches on center is most robust (more rebar, more cost, very strong). 18 inches is standard for residential slabs and balances cost and strength. 24 inches is economical but weaker.

Step 4: Specify whether you're placing rebar in one layer (typical for thin slabs under 4 inches) or two layers (thicker slabs, higher-load applications). Two layers nearly doubles the rebar required.

Step 5: The calculator shows total linear feet and the number of bars to order (rebar is sold in 20–60-foot lengths, depending on the supplier and size).

The Formula Behind the Math

Rebar calculation is based on grid spacing:

Linear Feet (One Direction) = (Slab Length ÷ Spacing) × Slab Width

Total Linear Feet = (Length Direction + Width Direction) × Number of Layers

Let's work through a concrete driveway example: 12 feet wide × 20 feet long, #4 rebar, 12-inch spacing, single layer.

#4 rebar comes in 20–60-foot lengths. You'd order roughly 9–10 bars of 60-foot lengths, or 26–27 bars of 20-foot lengths. Suppliers cut to length; confirm with them.

Our calculator handles this instantly.

Residential Driveway (4" Thick, 12" Spacing)

A typical driveway: 12 feet × 20 feet, 4 inches thick (single-layer rebar typical), #4 bars, 12-inch spacing.

This is a moderate amount. Cost is roughly $5–8 per linear foot, so $2,600–4,200 for material alone.

Large Foundation (12" Spacing, Two-Layer Grid)

A foundation slab 30 feet × 40 feet, 6 inches thick, #5 rebar, 12-inch spacing, two layers (top and bottom, standard for foundation slabs).

A foundation this size needs roughly 5,300 linear feet of #5 rebar. At 60-foot lengths, that's 88–90 bars. Cost is roughly $8–12 per linear foot = $42,000–63,600 for rebar alone. Professional contractors tie and place this.

Economic Patio (18" Spacing, Single Layer)

A budget-conscious patio: 16 feet × 16 feet, 4 inches thick, #4 rebar, 18-inch spacing (wider spacing = less material cost).

An economical patio like this needs roughly 376 linear feet of #4 rebar, or about 6–8 bars (60-foot lengths). Cost is roughly $1,900–3,000 for rebar.

Tips and Things to Watch Out For

Rebar spacing is regulated by building code-confirm minimums before ordering. Standard residential: 12 or 18 inches on center. Some jurisdictions allow 24 inches for light-duty slabs. Check your local code. Undersizing (spacing too far apart) can lead to cracking and inspection failure.

Two-layer grids are standard for foundations and thick slabs. Thin slabs (3–4 inches, patios, driveways) often use one layer (rebar in the middle of the slab). Thick slabs (6+ inches, foundations) use two layers (rebar near top and bottom surfaces). Confirm with building code or your structural engineer.

Rebar overlaps must meet code minimums-typically 24–40 bar diameters. A #4 bar (0.5-inch diameter) needs 12–20 inches overlap. Our calculator includes standard overlap; confirm with your engineer if loads are unusual.

Always order 10–15% more material than your calculation to account for waste, cuts, breakage, and measurement errors. Rebar is cut and bent on-site. Some bars break during transport or installation. Order the calculated amount plus 10–15% extra.

Tie wire or rebar chairs are separate from rebar material. Rebar chairs (plastic or metal supports) hold rebar at the correct height during concrete pouring. Tie wire (soft steel wire) ties rebar intersections together. Budget 50–100 chairs and several rolls of tie wire for typical slabs.

Placement depth matters for structural integrity. For slabs, rebar should sit roughly 2 inches from the top surface (for 4-inch slabs, that's the center or slightly lower). For foundations, rebar should be 2–3 inches from both top and bottom surfaces. Poor placement means the rebar isn't protecting the concrete where it matters most.

Frequently Asked Questions

How much rebar do I need for a 100 square foot slab?

At 12-inch spacing, single layer: roughly 200–240 linear feet of #4 rebar. At 18-inch spacing: roughly 135–160 linear feet. A 10×10 slab needs about 200–240 linear feet; a 15×7 slab is similar.

What's the difference between #4, #5, and #6 rebar?

Rebar sizes: #4 (0.5" diameter, 0.668 lbs per foot), #5 (0.625" diameter, 1.043 lbs per foot), #6 (0.75" diameter, 1.502 lbs per foot). Larger = stronger but heavier and more expensive. #4 and #5 are standard residential; #6 is for heavy-load applications.

Should I use 12-inch or 18-inch spacing?

12-inch spacing is stronger and better for freeze-thaw climates or areas prone to cracking. 18-inch spacing is standard residential and cost-effective. Use 12-inch if the slab is thin (under 4 inches), in harsh climate, or under frequent heavy loads. Use 18-inch for standard patios and driveways.

Can I skip rebar and use plastic reinforcement mesh instead?

Wire mesh (wire fabric) is cheaper and easier to lay than rebar but provides less reinforcement strength. It's acceptable for thin (3-inch) patios or decorative slabs that won't bear heavy loads. For driveways, foundations, or slabs over freezing climates, rebar is superior.

How much does rebar cost?

#4 rebar: $5–8 per linear foot. #5 rebar: $7–11 per linear foot. Prices vary by region and market. A 20-foot bar of #4 costs roughly $100–160. Budget accordingly when calculating total project cost.

Do I need an engineer to design my rebar layout?

For simple residential slabs (patios, driveways under 4 inches): building code tables often specify spacing and size. For foundations, thick slabs, or unusual loads: hire a structural engineer to verify design. This costs $300–1,000 but ensures code compliance and longevity.

How do I place rebar correctly during the pour?

Lay rebar grid on supports (rebar chairs) so it sits at the correct height (roughly 2 inches from top for a 4-inch slab). Tie grid intersections with tie wire to prevent shifting during concrete pour. The concrete should fully encase the rebar. Poor placement leads to weak concrete and corrosion.

Can rebar rust in concrete?

Good concrete (proper mix, adequate cover) protects rebar from rust. Poor concrete (high water-cement ratio, thin cover) allows water and air to reach rebar, causing corrosion and spalling. Proper design, placement, and concrete quality prevent this. Rebar chairs ensure adequate concrete cover.

Related Calculators

Use the concrete calculator to estimate concrete volume for your slab. The square footage calculator measures slab area precisely. The material cost estimator budgets rebar, concrete, finishing, and labor for the complete project.