Need total steel area, unit weight, or grade data for design? See the Rebar Size Chart for the full master reference, the Rebar Area Chart for cross-sectional area, and the Rebar Weight Chart for total weight calculations.

ASTM defines nominal dimensions for deformed bars by the dimensions of a plain round bar having the same weight (mass) per unit length as the deformed bar. This is one of the most important concepts on this page, because it means nominal diameter is a calculated design equivalent, not a direct physical measurement of a deformed bar’s irregular cross-section.

Nominal Diameter

The standardized design diameter used for engineering calculations, bar identification, reinforcement detailing, development-length equations, spacing checks, and cover calculations. This is the value published in ASTM A615/A615M Table 1 and the value shown throughout this chart.

Outside Diameter Over Deformations

A real deformed bar can measure larger across its ribs than its listed nominal diameter. Detailing references have long noted that the overall diameter measured across deformations is somewhat greater than nominal diameter, which can matter where physical clearance is critical, such as tight rebar cages or congested connections. See Nominal vs Actual Rebar Diameter below.

Values verified against ASTM A615/A615M Table 1 standard bar dimensions, including the current inclusion of the No. 20 [64] bar. See Why #9 and Larger Bars Break the Eighth-Inch Rule for the reason #9 through #18 don’t fit the simple fraction pattern.

This rule holds exactly for #3 through #8: #3 = 3/8″, #4 = 4/8″ = 1/2″, #5 = 5/8″, #6 = 6/8″ = 3/4″, #7 = 7/8″, #8 = 8/8″ = 1″. This is the most useful shortcut for common bar sizes, but it should not be extended blindly to bar #9 and larger.

The numbering history for larger U.S. bars relates to equivalent cross-sectional area rather than simply extending the diameter-in-eighths convention. Bar sizes #9, #10, #11, #14, and #18 were originally sized to match the weight and cross-sectional area of 1 in, 1-1/8 in, 1-1/4 in, 1-1/2 in, and 2 in square bars respectively, not to a diameter equal to the bar number in eighths of an inch.

#9, #10, and #11

#14 and #18

No. 20 [64]: Current ASTM Standard Note

Nominal Diameter

The standard design value published in ASTM A615/A615M Table 1, used for all engineering calculations.

Core / Equivalent Diameter

The equivalent plain-round bar dimension associated with the bar’s standardized nominal area and mass per unit length. This is functionally the same value as nominal diameter, defined by weight equivalence rather than a direct caliper measurement.

Overall Diameter Across Ribs

The physical dimension measured over the deformation peaks with a caliper. A deformed rebar cross-section is not a perfectly smooth circle, so a caliper placed across the ribs may not equal the ASTM nominal diameter. Detailing references have specifically warned that overall diameter can matter in clearance-sensitive detailing, such as tightly spaced cages or congested splice zones.

ASTM A615 includes deformation requirements alongside nominal bar diameter, covering transverse deformations, longitudinal ribs, rib height, rib spacing, and gap requirements. These deformations are what allow the bar to bond mechanically with surrounding concrete, but they also mean the physical outside dimension varies around the bar’s circumference and along its length.

ASTM specifies limits for average deformation spacing and minimum deformation height for each bar size, ensuring adequate bond performance across all manufacturers. This chart does not reproduce the full ASTM deformation table, since that level of detail is secondary to the nominal diameter identification this page focuses on; the key takeaway for readers is that rib geometry is the physical reason measured diameter differs from nominal diameter.

Common tools include a vernier caliper, a digital caliper, a tape measure or ruler for rough identification, comparison against a known-size bar, and checking rolled bar markings.

Measuring Across the Ribs

Placing calipers across the deformation peaks gives the overall outside diameter, not the ASTM nominal diameter. This measurement will typically read somewhat larger than the chart values above, which is expected and does not mean the bar is oversized or nonconforming.

Measuring an Unknown Bar

Combine the measured dimension with other identification clues: rolled bar markings, comparison against the nominal diameter chart, weight per foot if a measured length and weight are available, and the project drawings or specification, which govern the actual bar that should be installed.

1. Clean a short section of the bar to remove rust, coating buildup, or debris.
2. Look for rolled size markings, if visible.
3. Measure outside diameter with a caliper.
4. Compare the measurement against the nominal diameter chart above.
5. Account for rib height when interpreting the measured value.
6. Cross-check weight per foot if the bar size is still uncertain.
7. Verify against project drawings or specifications before substituting any bar size.

ASTM specifications require rolled identifying marks on deformed bars, generally including a mark identifying the mill or producer, a mark identifying the bar size, a mark identifying the steel type, and a mark or symbol identifying the grade.

CRSI detailing guidance notes that these rolled marks are the primary way to confirm a bar’s producer, size, steel type, and grade without relying solely on a physical diameter measurement, which is especially useful since diameter alone cannot distinguish between different grades of the same bar size.

For #4, 0.500 in × 25.4 = 12.7 mm exactly. The ASTM metric designation [13] is a rounded designation number, not a claim that the exact physical diameter is 13.0 mm. The nominal physical diameter remains 12.7 mm. Keeping this distinction clear prevents confusion between a bar’s designation label and its actual tabulated dimension.

ASTM soft-metric designations represent the same U.S. bar family (the identical #3 through No. 20 [64] bars) labeled with a metric designation number in brackets, such as #10 [32]. These are not separate bars; they are the same ASTM bar with a metric-style label.

True metric reinforcement systems used in other countries manufacture bars with nominal diameters such as 8 mm, 10 mm, 12 mm, 16 mm, 20 mm, 25 mm, 32 mm, or 40 mm. These round-number metric bars are governed by different national or international standards and should not be assumed automatically interchangeable with U.S. ASTM bars, even when a diameter happens to look similar.

Because area scales with diameter squared, doubling a bar’s diameter roughly quadruples its cross-sectional area rather than simply doubling it. For the full area-per-bar and area-per-spacing reference tables, see the dedicated Rebar Area Chart.

Weight per foot rises with the bar’s cross-sectional area, so it follows a similar squared relationship to diameter rather than a simple linear one. This page includes only enough weight data to help identify an unknown bar; for complete unit-weight and total-weight tables, see the dedicated Rebar Weight Chart.

A #5 Grade 60 bar and a #5 Grade 80 bar have the identical nominal diameter of 0.625 inches, but different mechanical properties and different specified minimum yield strengths. ASTM A615/A615M currently includes Grade 40 [280], Grade 60 [420], Grade 80 [550], and Grade 100 [690] bars.

Larger bars affect required clear spacing, congestion within a rebar cage, the number of bars needed to meet a required steel area, and how easily concrete can be placed and consolidated around the reinforcement. This page covers diameter’s role in these considerations only at a conceptual level; for full spacing tables and requirements, see the Rebar Spacing Chart.

Concrete cover is measured relative to the reinforcement surface, while bar diameter affects where the bar centerline sits within the member. Larger bars can therefore influence cage dimensions, clear cover geometry, layer spacing when multiple bar layers are used, and the concrete space available around each bar. See the Concrete Cover Chart for cover requirements by exposure condition and member type.

The nominal bar diameter, commonly written as d_b, appears directly in many development-length and lap-splice equations used in reinforced concrete design. Larger bars can require longer development or lap dimensions, depending on bar size, grade, concrete strength, cover, spacing, coating, confinement, and the applicable design provisions. This page does not reproduce the numerical lap-splice tables; see the dedicated Rebar Lap Splice Chart for those values.

Many detailing standards express minimum bend diameters as a multiple of the bar’s nominal diameter d_b, such as a minimum bend diameter of 6d_b for certain bar sizes. ASTM’s own bend-testing requirements likewise use bar diameter d in defining the bend-test pin diameters used to verify a bar’s ductility during manufacturing quality control.

Keep these two concepts separate: design bend-diameter requirements used in detailing drawings are not the same as the ASTM manufacturing bend test, even though both are expressed relative to bar diameter.

Slabs and Driveways

Residential slabs and driveways commonly use smaller bar sizes toward the lower end of the standard range.

Footings and Foundations

Footings and foundations often use mid-range bar sizes, depending on load and soil conditions.

Walls

Reinforced walls may use a range of bar sizes depending on wall height, loading, and reinforcement layout.

Beams and Columns

Beams and columns frequently use mid-to-large bar sizes to meet longitudinal reinforcement and confinement requirements.

Bridges and Heavy Structural Work

Bridges and other heavy structural elements may use the largest standard bar sizes, including #14, #18, and where recognized by the applicable code, No. 20 [64].

Inputs that go into selecting rebar diameter include required steel area, bar spacing, member dimensions, clear spacing, concrete cover, development length, congestion, constructability, loading and design requirements, and the engineer’s reinforcement schedule.

Replacing one bar size with another changes cross-sectional area, the number of bars needed, spacing, development and lap requirements, congestion, crack distribution, anchorage, and potentially the design strength of the member.

ASTM A615/A615M covers deformed and plain carbon-steel bars for concrete reinforcement in cut lengths and coils. Its scope includes standard bar sizes, number designations, nominal dimensions, grades, deformation requirements, and both inch-pound and SI unit systems.

The standard sizes and dimensions of deformed bars and their number designations are given in Table 1 of the specification. Current editions include Grade 40 [280], Grade 60 [420], Grade 80 [550], and Grade 100 [690] as the four minimum yield strength levels covered.

No. 20 [64]: Current Standard Note

Diameter and size references may also appear with ASTM A706/A706M low-alloy weldable reinforcement, stainless steel reinforcement specifications, coated reinforcement (such as epoxy-coated or galvanized bar), and proprietary reinforcement systems. This page does not attempt to be a complete rebar-type specification article.

This chart focuses on diameter identification and measurement. For full size, area, weight, grade, and spacing data, use these dedicated references.