Rebar Calculator: Bars, Weight and Spacing for Concrete Slabs and Footings

1. Enter the slab length and width in feet.
2. Select a rebar size from #3 to #8. Residential slabs typically use #4 rebar.
3. Set the spacing in inches. 12-inch spacing is standard for most residential slabs.
4. Choose 1 or 2 layers. A single layer is common; thicker slabs or footings may need two mats (top and bottom).
5. Enter the stock bar length (usually 20 ft) so the calculator can tell you how many bars to buy.
6. Read the results: total linear feet, weight, stock bars needed, and tie wire.

Rebar (reinforcing bar) is a steel bar embedded in concrete to resist tensile forces. Concrete handles compression well but is weak in tension. Steel rebar bridges that gap, preventing cracks and structural failure under load.

Most residential slabs, driveways, footings and foundation walls use rebar in a grid pattern. The bar size, spacing and number of layers depend on the load the concrete must carry and local building codes. Common setups include #4 bars at 12-inch spacing for residential slabs and #5 bars at 6-inch spacing for heavier structures.

For related concrete work, see the concrete calculator to estimate yards of concrete needed.

The calculator places bars in a grid pattern across a rectangular slab, counts them, and adds up the total length and weight.

1. Divide the slab width by the spacing to find how many lengthwise bars fit, then add 1 for the starting bar.
2. Do the same across the length for widthwise bars.
3. Multiply each group by the slab dimension they span to get linear feet, then sum both directions.
4. Multiply by the ASTM A615 weight per foot for the chosen bar size.
5. Divide each bar length by the stock bar length and round up to find how many stock bars to order.
6. Count intersections (lengthwise bars times widthwise bars) and multiply by 8 inches for tie wire.

Why Add 1 to the Bar Count

A grid needs a bar at each edge. If a 15-foot width is divided into 1-foot spaces you get 15 spaces and 16 bars (one at 0 ft, one at 1 ft and so on up to 15 ft).

Take a 20 x 15 ft slab with #4 rebar at 12-inch spacing, a single layer, and 20-ft stock bars.

Lengthwise bars (across the 15-ft width): floor(15 x 12 / 12) + 1 = 16 bars, each 20 ft long.

Widthwise bars (across the 20-ft length): floor(20 x 12 / 12) + 1 = 21 bars, each 15 ft long.

Total linear feet: (16 x 20) + (21 x 15) = 320 + 315 = 635 ft.

Weight: 635 x 0.668 = 424.18 lbs (192.4 kg).

Stock bars: 16 bars at 20 ft need 16 stock bars; 21 bars at 15 ft need 21 stock bars. Total: 37 stock bars.

Intersections: 16 x 21 = 336. Tie wire: 336 x 8 / 12 = 224 ft.

ASTM A615 is the most common specification for carbon-steel reinforcing bars in the United States. The bar number equals its diameter in eighths of an inch.

Weights are per linear foot. To convert to metric, multiply lb/ft by 1.4882 to get kg/m.

Spacing is the center-to-center distance between parallel bars. Closer spacing means more bars and a stronger grid, but also more material cost and labor.

Local building codes may specify minimum bar sizes and maximum spacing for structural elements. Always check your local requirements.

A single layer (one mat) is placed near the bottom of a slab to handle the main tensile stresses. A double layer adds a second mat near the top, which helps resist cracking from temperature changes and load reversals.

When to Use a Single Layer

Most residential slabs 4 to 5 inches thick use a single layer of #4 rebar at 12-inch spacing. This covers typical garage, driveway and patio loads.

When to Use Two Layers

Thicker slabs (6 inches and above), elevated slabs, footings carrying heavy point loads, and any slab designed by an engineer often call for two layers. The top mat is usually placed 1.5 to 2 inches below the top surface, and the bottom mat 1.5 to 2 inches above the bottom.

When a slab dimension exceeds the stock bar length (typically 20 ft), bars must be spliced by overlapping. ACI 318 requires a minimum lap splice of at least 24 bar diameters for tension splices in most cases.

This calculator counts stock bars assuming each bar is cut from one stock length with no splice overlap added to the bar length. If your project requires splices, add the overlap length to each bar that needs one and recalculate, or add extra stock bars as a buffer.

Tie wire holds the rebar grid together during concrete placement. A short piece of 16-gauge annealed wire is twisted around each intersection to keep the bars from shifting.

How Much Tie Wire Do You Need?

A common rule of thumb is about 8 inches (roughly 20 cm) of wire per intersection. The calculator multiplies the number of grid intersections by 8 inches and converts to feet.

Buying Tie Wire

Tie wire is usually sold in 3.5-pound coils. A 3.5-lb coil of 16-gauge wire contains roughly 340 feet. Divide the total tie wire footage by 340 to estimate how many coils to buy.

Rebar is typically sold by the foot or by weight. Knowing the total linear footage and weight from this calculator lets you get a quick price estimate from your local supplier.

Typical Price Ranges

Prices vary by region, bar size and market conditions. As a rough guide, #4 rebar often runs between $0.40 and $0.80 per linear foot, and #5 between $0.60 and $1.10. Always get a current quote from your supplier.

What Else to Budget For

Add tie wire, bar chairs or supports (to keep rebar at the correct height), and labor. Tie wire and chairs are modest costs, but labor for cutting, bending and placing rebar can be significant on larger projects.

Comparing Rebar to Welded Wire Mesh

For light-duty slabs, welded wire mesh (WWM) is a lower-cost alternative. It is faster to place but offers less strength and crack control than a proper rebar grid. Most structural slabs and footings still require rebar per the engineer's design.

Several factors change how much rebar a project needs beyond the basic grid calculation.

Slab Thickness

Thicker slabs often require larger bar sizes or tighter spacing. A 4-inch residential slab typically uses #4 bars, while an 8-inch commercial slab may need #5 or #6 bars.

Load Requirements

Heavy vehicle traffic, equipment loads or soil conditions can push the design toward closer spacing or a second layer of rebar.

Soil and Climate

Expansive soils, freeze-thaw cycles and high water tables can all increase the rebar needed to prevent cracking.

Openings and Edges

Slab penetrations for plumbing, HVAC or columns require extra bars around the opening to redistribute stresses. Slab edges and construction joints also need additional reinforcement.

Planning a Residential Slab

Before pouring a garage floor, driveway or patio, use the calculator to estimate bars, weight and stock bar count for your material order.

Estimating Footing Reinforcement

Footings for walls, columns and decks use the same grid layout. Enter the footing dimensions to get a quick quantity check.

Budgeting a Concrete Project

Knowing the total weight lets you price the rebar at your local supplier. Pair the result with the concrete calculator for a complete material estimate.

Verifying a Contractor Bid

Run the numbers yourself to confirm that a quoted rebar quantity is reasonable for the slab size and spacing specified in the plans.

1. Forgetting the Edge Bar

A common error is dividing the dimension by the spacing without adding 1. A 15-foot width at 12-inch spacing needs 16 bars, not 15.

2. Ignoring Splice Overlap

If any bar is longer than the stock length, you need a splice. Failing to account for the overlap means ordering too few bars.

3. Using the Wrong Bar Weight

Each bar size has a specific weight per foot. Using the weight for #4 when you actually need #5 underestimates the total weight by about 36 percent.

4. Omitting Tie Wire

Tie wire is easy to forget. Without enough wire, the grid shifts during the pour and the rebar ends up in the wrong position.

5. Not Checking Local Codes

Building codes specify minimum bar sizes, maximum spacing and required cover. Always check local requirements before finalizing your order.

The bar weights are exact ASTM A615 values and the geometry is straightforward arithmetic, so the numbers are precise for the inputs you provide.

Count the bars in each direction: divide the slab width by the spacing and add 1 for lengthwise bars, then divide the length by the spacing and add 1 for widthwise bars. Add both totals together. For a 20 x 15 ft slab at 12-inch spacing, that is 16 + 21 = 37 bars in a single layer.

Most residential slabs 4 to 5 inches thick use #4 rebar (1/2-inch diameter) at 12-inch spacing. Heavier slabs, footings and structural elements may need #5 or #6 bars. Check your local building code or consult an engineer.

Multiply the total linear feet of rebar by the ASTM A615 weight per foot for your bar size. For example, 635 feet of #4 rebar weighs 635 x 0.668 = 424.18 pounds.

12-inch center-to-center spacing is the most common for residential slabs. Heavy-duty slabs may use 6-inch spacing, while light-duty patios and sidewalks sometimes use 18-inch spacing.

Standard rebar is sold in 20-foot lengths. If a slab dimension exceeds 20 feet, bars must be spliced with an overlap of at least 24 bar diameters per ACI 318.

Plan for about 8 inches of 16-gauge tie wire per grid intersection. For 336 intersections, that is 224 feet of wire. A standard 3.5-pound coil holds about 340 feet.

The bar number is the diameter in eighths of an inch. A #4 bar is 4/8 or 1/2 inch in diameter. A #8 bar is 8/8 or 1 inch in diameter.

A single layer is standard for most residential slabs. Two layers (top and bottom mats) are used for thicker slabs, elevated slabs, and heavily loaded footings as specified by an engineer.

Use the same grid formula. Enter the footing length and width, choose the bar size and spacing from your plans, and the calculator gives you bars, weight and stock bar count.

The calculator uses imperial units and ASTM bar designations. For metric rebar (6 mm, 10 mm, etc.), convert your slab dimensions to feet, use the closest ASTM size, and note that the weight output includes both pounds and kilograms.

• ASTM A615 / A615M - Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement (ASTM International; bar designations, diameters and unit weights used in this calculator).
• ACI 318 - Building Code Requirements for Structural Concrete (American Concrete Institute; splice lengths, cover requirements and reinforcement detailing).
• CRSI - Concrete Reinforcing Steel Institute (Industry resource for rebar specifications, placement guidelines and construction practices).