Rebar Sizes & Areas Reference

Reference rebar size and area tables for preliminary concrete detailing. Confirm your jurisdiction's bar set.

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.

FAQ

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.

Related pages

• Concrete footing calculator
• Retaining wall calculator
• Unit converter
• Tools directory
• Reference tables directory
• How to verify calculator results
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Disclaimer (educational use only)

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