🧱 Rebar Calculator: Slab, Footing, Wall, Weight, and Cost
Slab, wall, footing & column rebar estimate — quantity, weight, lap splice, and cost
✓ Slab Grid ✓ Wall Rebar ✓ Weight & Cost ✓ Lap Splice| Bar Size | Dia. (in) | Dia. (mm) | Area (in²) | lbs/ft | kg/m | Common Use |
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This rebar calculator covers four use cases in one tool: concrete slab grid planning, wall and footing rebar, weight calculation by bar size, and a full size reference chart. Enter slab dimensions, pick a bar size and spacing, and get bar count, linear footage, total weight, and estimated cost in one step.
The calculator supports #2 through #18 bar sizes, lap splice lengths, edge clearance, waste allowance, and pricing by bar or by linear foot. The multi-piece list on Tab 2 handles wall and footing rebar separately from the slab.
Below the tool you will find how to calculate rebar for any structure, which bar size to use for each application, a rebar weight and size reference table, and answers to the questions most rebar pages skip.
What Is Rebar and Why Concrete Needs It
Rebar, short for reinforcing bar, is deformed steel rod embedded in concrete before it sets. The surface deformations (ridges or lugs) improve adhesion between the steel and the concrete matrix.
Concrete is strong in compression but weak in tension. Under load, a plain concrete slab cracks from the bottom up as tensile stress builds. Rebar carries the tension load that concrete cannot, distributing stress across the steel grid and preventing cracks from forming or widening.
- Tensile strength: rebar handles the pulling forces concrete fails under.
- Crack resistance: steel reinforcement keeps cracks narrow and contained.
- Reduced thickness: a reinforced slab can be thinner than an unreinforced one for the same load rating.
- Long-term durability: reinforced slabs and foundations outlast unreinforced pours by decades.
Concrete driveways, patios, foundations, retaining walls, columns, beams, and grade beams all rely on rebar to meet structural requirements. For most residential and commercial work, using unreinforced concrete is no longer considered acceptable practice.
How to Calculate Rebar for a Concrete Slab
Rebar in a slab runs in a grid pattern: one layer of parallel bars in one direction, crossed by another layer in the perpendicular direction. The grid sits inside the slab with a small gap (cover) between the rebar and each edge.
The calculation follows four steps:
- Grid dimensions: subtract the edge clearance from each slab dimension. A 20 ft slab with 3-inch clearance on each side has a grid length of 19.5 ft.
- Bar count: divide each grid dimension by the spacing, round up, and add 1 for the final edge bar.
- Linear footage: multiply bars in each direction by the opposite grid dimension, then sum both directions.
- Bar pieces: divide total linear footage by the stock bar length and round up to the next whole bar.
Example: a 20 ft × 15 ft slab, #4 rebar at 12-inch spacing, 3-inch edge clearance, 20 ft stock bars.
- Grid: 19.5 ft × 14.5 ft
- Bars across width: floor(19.5 / 1) + 1 = 21 bars, each 14.5 ft long
- Bars across length: floor(14.5 / 1) + 1 = 16 bars, each 19.5 ft long
- Total linear feet: (21 × 14.5) + (16 × 19.5) = 304.5 + 312 = 616.5 ft
- Bar pieces: ceil(616.5 / 20) = 31 bars
- With 10% waste: 35 bars
Rebar Spacing Guidelines by Project Type
Spacing is always measured center-to-center between bars, not edge to edge. The project type and local building code determine the correct spacing. Common ranges:
| Application | Bar Size | Typical Spacing |
|---|---|---|
| Patio, sidewalk, light slab (4 in.) | #3 | 18 to 24 in. |
| Residential driveway (4 to 5 in.) | #3 or #4 | 12 to 18 in. |
| Residential foundation slab | #4 or #5 | 12 in. |
| Residential wall (vertical) | #4 | 12 to 24 in. |
| Residential wall (horizontal) | #4 | 24 to 36 in. |
| Footing or grade beam | #5 | 12 to 18 in. |
| Commercial structural slab | #5 or #6 | Per engineer spec |
| Bridges, heavy industrial | #7 and above | Per engineer spec |
Always verify spacing with the project plans or a licensed structural engineer. ACI 318 sets minimum requirements, but local codes may impose stricter rules.
Edge Clearance and Concrete Cover
The gap between the rebar and the edge or surface of the concrete is called cover. Cover protects the steel from moisture and corrosion.
- Slab on grade: typically 3 inches at the bottom, 1.5 inches at the top.
- Walls exposed to weather: 2 inches minimum per ACI 318.
- Footings poured against soil: 3 inches minimum.
- Interior slabs not exposed to weather: 1.5 inches is common.
How to Calculate Rebar for a Concrete Wall
Wall rebar runs in two directions: vertical bars spaced along the wall length and horizontal bars (also called horizontal steel or wall ties) spaced up the wall height.
- Vertical bars: divide wall length by vertical spacing, add 1. Each bar is as tall as the wall height.
- Horizontal bars: divide wall height by horizontal spacing, add 1. Each bar spans the full wall length.
- Total linear feet: sum both directions and divide by stock bar length for piece count.
Example: 30 ft wall, 8 ft high. #4 bar. Vertical spacing 12 in., horizontal spacing 24 in., 20 ft stock bars.
- Vertical bars: 31 bars × 8 ft = 248 lf
- Horizontal bars: 5 bars × 30 ft = 150 lf
- Total: 398 lf. Bar pieces: ceil(398 / 20) = 20 bars.
Footings and Grade Beams
Footings typically run two or three continuous bars along the length of the footing, tied with stirrups at set intervals. For a simple continuous footing, the calculation is:
- Longitudinal bars: number of bars × footing length.
- Stirrups: (footing length / stirrup spacing) × perimeter of one stirrup.
The wall/footing tab in the calculator above handles the longitudinal bar count for straight footings. Stirrups require a separate count based on your design drawings.
Rebar Weight Calculation
Rebar weight depends on the bar size. Each size has a published weight per linear foot from CRSI (Concrete Reinforcing Steel Institute) standard tables.
The formula is: Weight (lbs) = Total linear feet × lbs/ft for that bar size.
For metric units: Weight (kg) = Total linear meters × kg/m for that bar size.
Rebar is often purchased by the ton at suppliers. Divide total weight in lbs by 2000 to get US short tons. Multiply by 0.4536 for metric tonnes.
Rebar Weight and Size Reference Table
| Bar Size | Diameter (in) | Diameter (mm) | Area (in²) | lbs/ft | kg/m |
|---|---|---|---|---|---|
| #2 | 0.250 | 6.35 | 0.05 | 0.167 | 0.249 |
| #3 | 0.375 | 9.53 | 0.11 | 0.376 | 0.560 |
| #4 | 0.500 | 12.70 | 0.20 | 0.668 | 0.994 |
| #5 | 0.625 | 15.88 | 0.31 | 1.043 | 1.552 |
| #6 | 0.750 | 19.05 | 0.44 | 1.502 | 2.235 |
| #7 | 0.875 | 22.23 | 0.60 | 2.044 | 3.042 |
| #8 | 1.000 | 25.40 | 0.79 | 2.670 | 3.973 |
| #9 | 1.128 | 28.65 | 1.00 | 3.400 | 5.059 |
| #10 | 1.270 | 32.26 | 1.27 | 4.303 | 6.403 |
| #11 | 1.410 | 35.81 | 1.56 | 5.313 | 7.906 |
| #14 | 1.693 | 43.00 | 2.25 | 7.650 | 11.384 |
| #18 | 2.257 | 57.33 | 4.00 | 13.600 | 20.237 |
Source: CRSI standard bar dimensions. The weight and size calculator on Tab 3 uses these values for any total rebar length you enter.
Rebar Bar Sizes Explained
In the US imperial system, the bar size number indicates the diameter in eighths of an inch. #3 = 3/8 inch = 0.375 in. #8 = 8/8 inch = 1.000 in. For sizes #9 through #18, the diameter no longer follows the eighths rule exactly but the naming convention continues.
Metric equivalents are expressed in millimeters of nominal diameter. Canadian metric bar sizes use a separate designation: 10M, 15M, 20M, 25M, 30M, 35M, 45M, 55M.
Lap Splice Length and Why It Matters
When a slab or wall dimension exceeds the length of a single stock bar, two bars must overlap (lap) to transfer load from one to the next. The overlap length is called the lap splice or development length.
The standard lap splice length is 40 times the bar diameter (40d). For higher-stress structural applications, 60 times the diameter (60d) is used. Always follow the project engineer’s specification.
- #3 bar: 40d = 40 × 0.375 in = 15 in lap.
- #4 bar: 40d = 40 × 0.500 in = 20 in lap.
- #5 bar: 40d = 40 × 0.625 in = 25 in lap.
The lap splice selector in Tab 1 adds the required extra length to the total linear footage automatically. If your slab dimension is within one stock bar length, no lapping is needed.
Types of Rebar by Material
Most projects use carbon steel rebar, which is the default in every calculator and cost table. Other types exist for corrosive environments:
- Carbon steel (Grade 60): the standard for residential and commercial concrete. Strong, workable, cost-effective.
- Epoxy-coated rebar: standard steel with a green epoxy layer. Used in bridge decks, parking garages, and coastal slabs to delay corrosion.
- Stainless steel rebar: maximum corrosion resistance. Used in marine structures, chemical plants, and coastal infrastructure. Significantly more expensive.
- GFRP (Glass Fiber Reinforced Polymer): non-magnetic and non-conductive. Used in MRI facilities, water treatment plants, and areas where steel corrosion is unacceptable.
Weight calculations apply directly to carbon steel and stainless steel rebar. GFRP rebar weighs significantly less: roughly 25 percent of steel rebar by volume. If you are using GFRP, do not use the steel weight table for material ordering.
Rebar Chairs and Concrete Cover
Rebar chairs (also called bar supports) hold the rebar grid at the correct height above the formwork or ground before the concrete is poured. The chair height determines the bottom cover of the slab.
Common chair heights for residential slabs: 1.5 in., 2 in., 3 in. Match the chair height to the required cover for your slab type. Placing rebar directly on the ground without chairs produces no cover and exposes the steel to moisture from below.
How to Use the Rebar Calculator
The calculator on this page has four tabs.
- Tab 1 (Slab): enter slab length and width, bar size, grid spacing, edge clearance, mat count, stock bar length, and lap splice setting. Optionally enter price per bar. Check the waste box to add 10%.
- Tab 2 (Wall/Footing): enter wall or footing dimensions, vertical spacing, horizontal spacing, bar size, and stock length. Results show vertical bars, horizontal bars, total pieces, and total weight.
- Tab 3 (Weight and Cost): enter bar size and total linear footage to get weight in lbs, kg, and US tons. Enter price per foot or price per ton for a cost estimate.
- Tab 4 (Reference): complete bar size table with diameter, area, lbs/ft, and kg/m. Highlight any bar size. Application guide for choosing the right bar size by project type.
Frequently Asked Questions
How do I calculate rebar for a concrete slab?
Subtract the edge clearance from each slab dimension to get the grid size. Divide each grid dimension by the spacing and add 1 for the total bars in each direction. Multiply bars by bar length to get linear footage. Divide by stock bar length for the piece count. Add 10% for waste.
What size rebar should I use for a concrete slab?
#3 rebar at 18 to 24 inches suits patios and sidewalks. #4 at 12 to 18 inches is standard for driveways and light footings. #5 at 12 inches is typical for residential foundations. Always confirm with the project engineer or local building code.
How do I calculate rebar weight?
Multiply total linear feet by the weight per foot for the bar size. #4 rebar weighs 0.668 lbs/ft. 100 linear feet of #4 = 66.8 lbs. Use Tab 3 of the calculator to handle this automatically for any bar size and any length.
What is the standard lap splice length for rebar?
40 times the bar diameter for standard applications. For a #4 bar (0.500 in. diameter): 40 × 0.500 = 20 inches. For structural or high-stress applications, 60 times the diameter may be required. Follow the project engineer’s specification.
How much rebar do I need per square foot of concrete?
At 12-inch spacing (the most common residential setting), you need approximately 2 linear feet of rebar per square foot of slab area (one bar each direction per square foot of grid). At 18-inch spacing, the figure drops to roughly 1.33 linear feet per square foot.
Should I use rebar or wire mesh for my driveway?
Rebar is preferred for driveways 5 inches thick or more. Wire mesh is adequate for 4-inch pours under light traffic loads. For any driveway that carries vehicles regularly, #3 or #4 rebar at 12 to 18 inches gives better crack resistance than mesh.
What is rebar development length?
Development length is the minimum length of rebar that must be embedded in concrete for the bond to transfer the full tensile force. It is calculated from bar diameter, concrete strength, and bar spacing per ACI 318. For typical Grade 60 rebar in 3000 psi concrete, development length is roughly 20 to 30 bar diameters depending on cover and spacing.
Can I weld rebar together instead of tying it?
Standard carbon steel rebar has a high carbon content that makes it brittle under the thermal stress of welding. Welded joints in rebar typically fail at lower tension than the design load. Use 16-gauge tie wire to fasten intersecting bars. Avoid welding unless the rebar is specifically designated as weldable (ASTM A706 Grade 60).
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- Square Footage Calculator — Measure slab or lot area for any shape
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Conclusion
Getting rebar right before the concrete pour saves time, money, and structural problems later. Measure the slab or wall, pick the correct bar size for the application, set the spacing, and let the rebar calculator handle the bar count, linear footage, weight, and cost in one step. Add the 10 percent waste allowance for cuts and offcuts, confirm the lap splice length if your dimension exceeds the stock bar length, and verify the final spec with your engineer or local building code before ordering.