How bar numbers work: #3 through #8 correspond to the bar diameter in eighths of an inch (#4 = 4/8 = 1/2 in). #9, #10, #11 derive from old square-bar equivalents (1 in, 1-1/8 in, 1-1/4 in squares). #14 and #18 derive from 1-1/2 in and 2 in square bars.

Most Commonly Used Sizes

Size Primary Application Stock Status
#3 Stirrups, ties, light slabs Readily available
#4 Slabs, walls, temperature/shrinkage Readily available
#5 Beams, columns, footings Readily available
#6 Heavy beams, pile caps Readily available
#8 Columns, heavy footings Readily available
#9 Large columns, transfer beams Special order
#11 Large columns, bridge caps Special order
#14–#18 Heavy civil, very large columns Limited availability

Grade Markings per ASTM A615

Rebar grades are identified by rolled-in markings on the bar surface:

Grade Yield Strength (ksi) Tensile Strength (ksi) Marking Color Code
Grade 40 40 60 None (plain) None
Grade 60 60 90 One line None
Grade 75 75 100 One line None
Grade 80 80 100 Two lines None
Grade 100 100 120 Three lines None

The grade marking consists of continuous longitudinal line(s) on the bar surface. Grade 60 (Fy = 60 ksi / 420 MPa) is the default for most US structural applications.

ASTM A706 (Low-Alloy Weldable Rebar)

ASTM A706 is specified when welding rebar or for seismic special moment frames:

Property A706 Grade 60 A615 Grade 60
Minimum Fy 60 ksi 60 ksi
Maximum Fy 78 ksi None
Fy/Fu Ratio ≤ 0.85 ≤ 0.89
Carbon Equivalent ≤ 0.55% Not specified
Weldability Designed for welding Not guaranteed

A706 is mandatory for welded rebar connections in ACI 318 Section 25.5.7. Use A706 when design requires rebar welding or when seismic ductility demands controlled yield properties.

Metric Rebar Equivalents

Canadian/Australian Bars (CSA G30.18 / AS/NZS 4671)

Designation Dia. (mm) Area (mm²) Mass (kg/m) Closest US Bar
10M 11.3 100 0.785 #3
15M 16.0 200 1.570 #5
20M 19.5 300 2.355 #6
25M 25.2 500 3.925 #8
30M 29.9 700 5.495 #9
35M 35.7 1000 7.850 #11
45M 43.7 1500 11.775 #14
55M 56.4 2500 19.625 #18

European Bars (EN 10080)

Diameter (mm) Area (mm²) Mass (kg/m) Grade
8 50.3 0.395 B500B
10 78.5 0.617 B500B
12 113.1 0.888 B500B
16 201.1 1.578 B500B
20 314.2 2.466 B500B
25 490.9 3.853 B500B/C
32 804.2 6.313 B500B/C
40 1256.6 9.865 B500C

Development Length per ACI 318-19

The development length ld is the minimum bar embedment needed to transfer full yield force into concrete:

ld = (3/40) × (fy / (λ√f'c)) × (ψt × ψe × ψs / ((cb + Ktr)/db)) × db

Simplified Straight Development Length — Grade 60 (inches)

For bottom bars, uncoated, normal-weight concrete, with (cb+Ktr)/db = 1.5:

Bar f'c = 3,000 psi f'c = 4,000 psi f'c = 5,000 psi f'c = 6,000 psi
#3 16 14 12 11
#4 21 19 17 15
#5 27 24 21 19
#6 32 28 25 23
#7 47 41 37 34
#8 54 47 42 38
#9 61 53 47 43
#10 69 59 53 48
#11 76 66 59 54
#14 92 80 71 65

Values use ψs = 0.8 for #6 and smaller, ψs = 1.0 for #7 and larger. Minimum ld = 12 in per ACI 318-19 Section 25.4.2. For top bars (more than 12 in of concrete below), multiply by 1.3.

Rebar Area Calculation for Flexural Design

The area of steel required in flexure per ACI 318-19:

As = Mu / (φ × fy × (d - a/2))

Where a = As × fy / (0.85 × f'c × b). Common rebar combinations:

Bar Size Number of Bars Total Area (in²)
#4 2 0.40
#4 4 0.80
#5 2 0.62
#5 4 1.24
#5 6 1.86
#6 4 1.76
#6 6 2.64
#7 4 2.40
#7 6 3.60
#8 4 3.16
#8 6 4.74
#9 4 4.00
#9 6 6.00
#10 4 5.08
#11 4 6.24
#11 6 9.36

Minimum flexural reinforcement (ACI 318-19 Section 9.6.1):

As,min = 3 × √f'c × bw × d / fy    (but not less than 200 × bw × d / fy)

For a 12-in wide beam with d = 20 in, f'c = 4,000 psi, fy = 60,000 psi:

As,min = 3 × √4000 × 12 × 20 / 60000 = 0.76 in²
200 × 12 × 20 / 60000 = 0.80 in² (governs)

This requires 2-#6 bars (As = 0.88 in²) or 3-#5 bars (As = 0.93 in²).

Rebar Weight Estimation

For material takeoffs and cost estimation:

Bar lb/ft ft/ton Tons per 1000 ft
#3 0.376 5,319 0.188
#4 0.668 2,994 0.334
#5 1.043 1,918 0.522
#6 1.502 1,332 0.751
#7 2.044 978 1.022
#8 2.670 749 1.335
#9 3.400 588 1.700
#10 4.303 465 2.152
#11 5.313 376 2.657
#14 7.650 261 3.825
#18 13.600 147 6.800

Installed rebar cost typically ranges $0.80–$1.50 per pound depending on bar size (larger bars are cheaper per pound) and project complexity.

Worked Example: Rebar Area Check

Problem: Verify that 4-#8 bars provide adequate tension reinforcement for a 12x18 in. concrete beam with Mu = 120 kip-ft, d = 15.5 in. fc' = 4,000 psi, Grade 60 rebar.

Given:

Solution:

Step 1 -- Depth of compression block (ACI 318-19 Section 22.2.2):

a = As * Fy / (0.85 * fc' * b)
  = 3.16 * 60 / (0.85 * 4 * 12)
  = 189.6 / 40.8
  = 4.65 in.

Step 2 -- Neutral axis depth:

c = a / beta1

beta1 = 0.85 (fc' = 4,000 psi, per ACI 318-19 Table 22.2.2.4.3)

c = 4.65 / 0.85 = 5.47 in.

Step 3 -- Strain check (tension-controlled):

epsilon_t = 0.003 * (d - c) / c = 0.003 * (15.5 - 5.47) / 5.47 = 0.00550 > 0.005

Tension-controlled: phi = 0.90 per ACI 318-19 Table 21.2.2.

Step 4 -- Nominal and design moment capacity:

Mn = As * Fy * (d - a/2)
   = 3.16 * 60 * (15.5 - 4.65/2)
   = 189.6 * 13.175
   = 2,498 kip-in = 208.2 kip-ft

phi*Mn = 0.90 * 208.2 = 187.4 kip-ft > 120 kip-ft  OK

Result: DCR = 120/187.4 = 0.64. The 4-#8 bars provide adequate flexural strength with ample reserve. Minimum reinforcement check: As_min = max(3sqrt(4000)/600001215.5, 20012*15.5/60000) = max(0.59, 0.62) = 0.62 in^2 < 3.16 in^2 OK.

Frequently Asked Questions

What is the cross-sectional area of a #5 rebar? A #5 bar has a nominal diameter of 0.625 in and a cross-sectional area of 0.31 in² (200 mm²). It is the most commonly used bar size for beams and general reinforcement in US construction.

What is the most common rebar size? #4 (0.500 in diameter, 0.20 in² area) for slabs and walls, #5 (0.625 in, 0.31 in²) for beams and footings, and #8 (1.000 in, 0.79 in²) for columns are the most commonly specified sizes.

What does the # in rebar sizing mean? The bar number (#3 through #8) corresponds to the diameter in eighths of an inch. A #4 bar is 4/8 = 1/2 in diameter. For #9, #10, #11, the numbers reference old square-bar equivalents. The area in in² equals the bar number divided by 10 for #3–#8 (e.g., #5 = 0.31 in² ≈ 5/16 = 0.3125).

What is the difference between ASTM A615 and A706 rebar? A615 is the standard specification for carbon-steel rebar. A706 is a low-alloy steel with controlled yield (60–78 ksi), maximum Fy/Fu ratio (≤0.85), and carbon equivalent limits, making it suitable for welding and seismic applications. A706 is required for welded rebar splices per ACI 318 Section 25.5.7.

How long is the development length for a #8 bar in 4000 psi concrete? For a bottom bar, uncoated, normal-weight concrete at f'c = 4,000 psi: ld = 47 in (approximately 4 ft). For a top bar (more than 12 in of concrete below), multiply by 1.3: ld = 61 in. Minimum ld = 12 in.

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Related Pages

Educational reference only. Development lengths depend on project-specific cover, spacing, and confinement. Always verify per ACI 318-19 or governing concrete code with a qualified engineer.