Rebar Size Chart — Bar Diameters, Areas & Weights (US #3–#18)

Cross-sectional area, diameter, and weight per foot for US (#3-#18), Canadian/Australian metric (10M-55M), and European (8-40 mm) reinforcing bars. Values per ASTM A615, CSA G30.18, and EN 10080. Confirm bar set with your project specification before design.

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US Rebar Sizes (#3–#18) — ASTM A615

Metric Rebar Sizes — Canadian/Australian (CSA G30.18, AS/NZS 4671)

European Rebar Sizes — EN 10080

Cross-System Comparison — Closest Matches

Understanding rebar naming and sizing conventions

Reinforcing bar designations are one of the most common sources of confusion in international structural engineering, because three completely different naming systems are in widespread use. In the United States, bars are designated by their diameter in eighths of an inch: a #4 bar is 4/8" = 0.500" diameter, a #8 bar is 8/8" = 1.000" diameter. This system extends from #3 (3/8") through #18 (2.257", which breaks the neat pattern because it was originally a 2-1/4" square bar converted to a round equivalent). In Canada and Australia, metric bar designations use the nominal diameter in millimeters: 10M, 15M, 20M, 25M, 30M, 35M, 45M, 55M. In Europe and much of Asia, bars are designated directly by their nominal diameter in millimeters with a diameter symbol: 8 mm, 10 mm, 12 mm, 16 mm, 20 mm, 25 mm, 32 mm, 40 mm.

The cross-sectional area of each bar is the single most frequently looked-up property in concrete design. Every flexural, shear, and development-length calculation depends on the total area of steel provided. Getting the area wrong -- whether by confusing US and metric designations or by misremembering a bar area -- directly corrupts the capacity calculation. A US #5 bar (Ab = 0.31 in2) and a Canadian 15M bar (Ab = 200 mm2 = 0.31 in2) happen to be nearly identical, but a US #6 bar (Ab = 0.44 in2) and a 20M bar (Ab = 300 mm2 = 0.465 in2) are not. These small differences propagate when multiplied across a group of bars.

Bar strength grades add another layer. In US practice, Grade 60 (fy = 60 ksi = 420 MPa) is the default for most structural applications, with Grade 80 and Grade 100 increasingly used for columns and high-seismic applications. Australian practice uses Grade 500N (fy = 500 MPa) for deformed bars, previously designated as Grade 400Y. European practice specifies B500B or B500C (fy = 500 MPa) with ductility classes B (normal) and C (high). These grades are not interchangeable across codes, because ductility requirements, elongation limits, and bend-test criteria differ.

Rebar selection checklist

When entering rebar data into any calculator or checking a detailing drawing, verify the following:

• Bar designation matches the project's code and jurisdiction. Do not mix US # designations with metric mm designations in the same calculation.
• Cross-sectional area corresponds to the correct bar. Look up the area; do not rely on memory. Common errors include confusing #5 (0.31 in2) with #6 (0.44 in2), or 16 mm (201 mm2) with 20 mm (314 mm2).
• Bar grade matches the specification. Grade 60 is the most common in US practice, but Grade 40, Grade 80, and Grade 100 bars exist. The grade determines fy for capacity calculations and affects development length requirements.
• Deformed vs. plain bars. Structural reinforcement uses deformed (ribbed) bars, which develop bond through mechanical interlock. Plain (smooth) bars are generally limited to spirals, ties, and specific detailing situations. Bond and development-length equations differ between deformed and plain bars.
• Availability in your market. Not all bar sizes are stocked everywhere. Large bars (#14, #18 in US, or 40 mm and above) may require special ordering and have longer lead times.
• Rebar in steel-to-concrete connections. Base plate and anchor bolt designs may involve rebar as supplementary reinforcement to enhance concrete breakout capacity. Ensure the rebar grade and development length are compatible with the anchorage design.

For the full verification and documentation workflow, see How to verify calculator results.

Frequently Asked Questions

What do the US bar numbers (#3 through #18) actually mean? Bar numbers #3 through #8 equal the nominal diameter in eighths of an inch. A #3 bar is 3/8" diameter, a #7 bar is 7/8" diameter. Starting at #9, the numbering corresponds to the former square-bar sizes that were replaced by round bars of equivalent area. #9 through #11 follow the same approximate pattern, but #14 and #18 are based on 1.693" and 2.257" diameter equivalents of the old 1-1/2" and 2" square bars.

How do I convert between US and metric bar designations? There is no exact 1:1 correspondence. A US #4 bar (12.7 mm diameter) is close to a 12 mm metric bar but not identical. A US #5 (15.9 mm) is close to a 16 mm bar. For cross-code comparison, use the actual diameter and area rather than assuming the designations are equivalent. The ASTM A615/A615M standard lists both imperial and metric properties for US bars.

What is the difference between Grade 60 and 500 MPa rebar? Grade 60 rebar has a specified yield strength of 60 ksi (approximately 414 MPa). Grade 500 rebar (used in Australian, European, and many Asian codes) has a specified yield strength of 500 MPa (approximately 72.5 ksi). They are not equivalent. Using Grade 500 rebar in a calculation designed for Grade 60 would overestimate the design yield by about 20%, which is non-conservative for ductility checks and over-strength calculations in seismic design.

What does "deformed" mean for rebar? Deformed bars have raised ribs or lugs rolled onto the surface during manufacturing. These ribs provide mechanical interlock with the surrounding concrete, which is essential for developing bond stress and transferring forces between the reinforcement and the concrete. Plain (smooth) bars rely on friction and adhesion alone, resulting in much lower bond capacity and longer required development lengths. All primary structural reinforcement uses deformed bars.

When does rebar appear in steel base plate design? Rebar appears in base plate design as supplementary reinforcement to improve concrete anchorage capacity. Hairpin bars, ties around anchor bolts, or headed reinforcement placed within the projected breakout cone can increase the concrete breakout strength and provide a ductile failure mode. When supplementary reinforcement is present and properly developed, some codes (e.g., ACI 318) allow higher strength reduction factors for the anchorage design.

What are the standard bar areas for US #3 through #8 rebar? The areas most commonly used in design are: #3 = 0.11 in², #4 = 0.20 in², #5 = 0.31 in², #6 = 0.44 in², #7 = 0.60 in², #8 = 0.79 in². These values should be memorized or looked up precisely — confusing #5 (0.31 in²) with #6 (0.44 in²) produces a 42% error in calculated steel area, which directly affects flexural capacity and development length calculations.

What is the minimum development length for a #5 Grade 60 bar in normal-weight concrete with f'c = 4000 psi? Per ACI 318-19 Section 25.5.2, the basic development length for a straight #5 deformed bar in tension (Grade 60, f'c = 4000 psi, uncoated, normal-weight concrete, ≥ 1d clear cover, ≥ 1.5d clear spacing) is ld = 24.0 in (24 bar diameters × 0.625 in/bar). With modification factors for excess steel area or enclosing transverse reinforcement, the required length can be reduced. Compression development lengths are shorter: approximately 12 in for the same conditions.

ASTM A615 vs ASTM A706 — Rebar Grade Comparison

ASTM A615 and ASTM A706 are the two most common rebar specifications in US practice. A615 is the default for general construction, while A706 is required where seismic ductility or controlled weldability is specified.

Use ASTM A706 when the project specification calls for seismic design categories D through F, when field welding of reinforcement is required, or when the engineer specifies controlled ductility and tight yield-strength limits. For gravity-only framing in low-seismic regions, ASTM A615 Grade 60 is standard.

Grade 60 vs Grade 80 — Properties and Design Implications

Higher-strength rebar grades allow engineers to reduce bar quantities or fit reinforcement into congested sections, but they come with trade-offs in ductility, crack control, and development length.

Grade 80 bars reduce the number of bars needed in columns and walls, which improves constructability in heavily reinforced members. However, ACI 318-19 limits the design yield strength used in flexural calculations to 80 ksi maximum (Section 20.2.2.4), and some provisions further cap the effective fy. Development and splice lengths scale proportionally with fy, so switching from Grade 60 to Grade 80 increases lap lengths by 33%, which can create congestion at splice locations. Always verify that the longer development lengths can be accommodated within the member geometry.

Metric Bar Sizes (10M–55M) — Detailed Properties

Metric designations used in Canada (CSA G30.18) and Australia (AS/NZS 4671) follow a different sizing philosophy than US bars. The number before the "M" approximates the nominal cross-sectional area in hundreds of square millimeters (e.g., 25M has approximately 500 mm² area). The following table provides expanded properties including perimeter and typical bend radii.

Bend pin diameters shown are typical minimum values per CSA A23.1 for Grade 400R bars. Tighter bends may cause cracking, especially in higher grades. For Grade 500 bars, increase the pin diameter by one bar diameter.

Common Rebar Applications by Structural Member

Choosing the right bar size for each member type is driven by constructability, required area of steel, and minimum cover requirements. The table below summarizes typical bar sizes, spacing, and grades for common reinforced concrete members.

This table reflects typical practice. Always size reinforcement based on calculated demand, code minimums, and constructability review. Seismic special systems may require ASTM A706 Grade 60 or 80 for all longitudinal reinforcement in special moment frames and special structural walls.

Cut-and-Bend Scheduling Basics

Cut-and-bend schedules (also called bending schedules or rebar fabrication lists) translate design drawings into shop instructions for the fabricator. Accurate scheduling prevents costly field errors and material waste. Key principles:

• Bar mark system. Each unique bar shape, size, and length receives a bar mark (e.g., "4H20-1" = #4 bar, shape code H20, mark 1). All bars with the same mark are identical and batched together during fabrication.
• Shape codes. Standard shape codes (ACI Detailing Manual, CRSI, or BS 8666 for international projects) define the bending pattern. Common shapes include straight bars (00), 90-degree hooks (01), 180-degree hooks (02), stirrups (T1), U-bars, and multi-bend shapes.
• Cutting lengths. The cutting length equals the sum of all straight segments plus the bend allowances. For a 90-degree bend, the bend allowance is approximately 0.3 × diameter × tan(45°) per bend, though fabricators typically use pre-calculated tables. Do not simply sum outside dimensions without accounting for bend take-up.
• Bend radius limits. Standard hooks per ACI 318 Table 25.3.1 specify minimum bend diameters: 6d for #3–#5 (180° hook), 6d for #3–#8 (90° hook), and 8d for #6–#8 (180° hook). Tighter bends risk cracking the bar, especially at larger diameters.
• Lap splice locations. The schedule must indicate where lap splices occur and their required length. In columns, splices typically fall just above the floor level. In beams, splices are located in regions of low stress (near inflection points) where possible.
• Material takeoff. The schedule tallies total weight per bar size for ordering. Fabricators order in 20 ft, 40 ft, or 60 ft mill lengths, and the cutting pattern should minimize offcut waste. Optimizing the nesting of cut lengths within stock lengths can reduce waste from 5–10% to under 3%.

Rebar Area per Foot of Width

For slab and wall design, engineers frequently need the total steel area per foot (or per meter) of width at a given bar size and spacing. This is calculated as As = Ab / s, where Ab is the single-bar area and s is the center-to-center spacing. The table below provides pre-calculated values for common US bar sizes.

Values shown are in square inches per foot of width (in²/ft). To convert to metric: multiply by 2119 to get mm²/m. For example, #5 at 8" spacing = 0.47 in²/ft × 2119 = 996 mm²/m.

Quick formula: As per foot = (Ab × 12) / spacing in inches. To find the spacing that provides a required area: spacing = (Ab × 12) / As required.

Development Length Quick Reference — ACI 318-19

Development length (ld) is the minimum embedment length required to fully develop the yield strength of a reinforcing bar in tension or compression. The following simplified values assume common conditions: normal-weight concrete, uncoated bars, with adequate cover and spacing (≥ 1db and ≥ 1.5db respectively).

Tension development length — simplified (ld)

Values are approximate and based on ACI 318-19 Section 25.4.2.2 (simplified method) with no modification factors applied. Actual development length may be shorter with confining reinforcement, excess steel area, or epoxy coating reduction factors (when applicable). Always verify against the full code provisions for your specific conditions.

Compression development length — simplified (ldc)

Compression development lengths are significantly shorter than tension values because bond resistance is enhanced by the bearing of bar ribs against the surrounding concrete under end bearing.

Compression development length is governed by the larger of (0.02 × fy / √f'c) × db and (0.0003 × fy) × db, with a minimum of 8 inches per ACI 318-19 Section 25.4.9. The values above include the 8-inch minimum floor.

For full development length calculations with all modification factors, use the rebar development length reference or the rebar calculator.

Related pages

• Concrete footing calculator
• Retaining wall calculator
• Unit converter
• Tools directory
• Reference tables directory
• How to verify calculator results
• Disclaimer (educational use only)
• Rebar size chart
• Rebar development length
• Rebar spacing chart

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Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

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