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Anchor Bolt Sizes: Diameter, Length, and Thread Specifications

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Anchor bolt sizes look simple on a schedule. In practice, every dimensional parameter directly impacts the rest of the anchor bolt set. A longer projection demands more thread, a higher grade changes the diameter a designer can justify, and a galvanized finish alters the nut. This article works through those dependencies for foundation anchors. Selecting the correct anchor bolt demands a clear understanding of sizing systems, international standards, and surface finishes. We’ve compiled key metric and imperial data to help engineers, site contractors, and buyers evaluate anchor bolt suppliers in UAE with confidence. That said, this resource covers cast-in-place structural anchors only; chemical capsules and expansion wedge anchors in 10 mm, 12 mm, and 16 mm diameters are excluded.

Anchor Bolt Size Chart in mm

Foundation anchor sizes are not one-size-fits-all. Instead, they cluster into four distinct diameter groups. Standard selection charts typically map precise anchor bolt dimensions in mm directly against project-required length. Look for your required anchor bolt length and diameter below to instantly align your base plate, grout allowance, and embedment depth.

Size Group
Diameter (mm)
Typical Lengths (mm)
Common Applications
Small (M6 – M12)
6 – 12
50 – 400
Light steelwork and railings
Medium (M16 – M30)
16 – 30
120 – 1000
Pipe supports and machinery
Large (M36 – M56)
36 – 56
300 – 2000
Heavy frames and towers
Industrial (M64 – M100)
64 – 100
600 – 3000+
Skyscrapers and bridge girders
  • Note: These values are for estimation only. Petrofast can manufacture any custom drawing as per the project’s requirement.

Selecting a Size Group

  • Small: Handrail posts, light brackets, and secondary steel. Loads are low, and embedment is usually shallow.
  • Medium: Pipe supports and machinery bases sit here. Vibration is the usual concern, which is why double nuts are often specified.
  • Large: Heavy frames and towers, where uplift and overturning determine both bolt count and diameter.
  • Industrial: Long embedment, tight alignment, and thick base plates. A rigid template matters more at this scale than at any other.

Imperial Sizes

Imperial anchor bolt sizes begin at 1/2 in and stop at 2 in in this range, threaded to the UNC series under ASTM F1554. Each converts to an awkward metric diameter, which matters when a drawing mixes unit systems.

Imperial Size
Equivalent (mm)
UNC Threads per Inch
1/2 in
12.7
13
5/8 in
15.9
11
3/4 in
19.1
10
7/8 in
22.2
9
1 in
25.4
8
1-1/4 in
31.8
7
1-1/2 in
38.1
6
2 in
50.8
4.5

Metric Thread Pitch and Plate Hole Data

Size
Coarse Pitch (mm)
Stress Area (mm²)
Clearance Hole (mm)
M12
1.75
84.3
14
M16
2.0
157
18
M20
2.5
245
22
M24
3.0
353
26
M30
3.5
561
33
M36
4.0
817
39

Engineers rely on coarse pitch profiles for metric anchor bolts. This geometry achieves two critical goals: maximizing shear resistance and accelerating on-site assembly engagement. Still, drawing boards don’t dictate the field. Although ISO/DIN specifications strictly govern standard clearance holes to accommodate thermal expansion and slight positioning tolerances, actual site conditions invariably produce subtle setting-out variances.

Shapes and Their Dimensional Differences

The shape determines the dimensions of the drawing. The guide to types of anchor bolts is followed by the dimensional points of selection.

  • L Anchor Bolt: Features a 90° L-shaped bend, with BL1 and R dimensions indicated on the drawing. There is no J depth, as the bend leg is straight.
  • J Anchor Bolt: Features a curved hook with a defined J depth, with SL1 and JID indicated on the drawing. There is no equivalent depth for an L anchor bolt.
  • Straight Anchor Bolt: Has no bend. The embedded end is secured using a nut and washer or an anchor plate.
  • Plate-Welded Anchor Bolt: A steel plate is welded to the embedded end of the bolt to provide anchorage.
  • T Anchor Bolt: Features a T-shaped head in place of a bent end to provide anchorage within the concrete.

Anchor Bolt Length And Thread Length

There is no standard rule for the length of anchor bolt threads. The project requirements and the bolt set are defined by the product’s custom drawing. However, the basic logic for calculating anchor bolt length remains similar:

  • Projection: Grout height + Base plate height + Washer height + Nut height + Allowance.
  • Allowance (Estimated): 25 mm to 50 mm to accommodate levelling requirements.
  • Straight Length: Embedment depth + Projection.

Worked example, M20.

The straight length is approximately 550 mm, based on a grout height of 30 mm, base plate thickness of 25 mm, washer thickness of 3 mm, nut height of 18 mm, and an embedment depth of 450 mm. If the anchor bolt has a bend leg, its dimensions are shown separately on the drawing.

The embedment depth is not a catalogue value. It depends on the tension and shear forces acting on the connection, the concrete strength, the edge distance, and the spacing between adjacent anchor bolts.

Estimated Weight Of Straight Shank Anchor Bolts (kg)

Weight per piece is used for freight planning and quantity checks. The approximate weight of a carbon steel anchor bolt can be calculated using the formula d² × L × 6.17 × 10⁻⁶ (kg), where d is the diameter and L is the length of the steel, both measured in millimetres. Austenitic stainless steel is approximately 2% heavier. The figures given below apply to the straight shank only and exclude any bend, nut, or washer.

Size
Length (mm)
Approx. Weight (kg)
M12
300
0.27
M16
400
0.63
M20
500
1.23
M24
600
2.13
M30
800
4.44
M36
1000
7.99
  • Note: The prescribed values are estimations based on insights of our engineers. Exact specifications may vary.

Anchor Bolt Grades and Standards

Anchor bolt grade determines yield and tensile strength, which in turn influence the diameter required to withstand a given load. Four standards cover most specifications. For more details on material selection, refer to our guide on anchor bolt grades and materials.

Standard
Grades
EN 10025-2
S275JR, S355JR
ASTM F1554
Grade 36, Grade 55, Grade 105
ISO 898-1
Class 4.6, 8.8, 10.9
ASTM A276
SS304, SS304L, SS316, SS316L, S31803, S32750, S32760
  • Grade 36: Suitable for general structural bases, Grade 36 is one of the most suitable ASTM F1554 grades for field bending and bending adjustments, subject to applicable requirements and engineering approval.
  • Grade 55: Designed to carry higher loads than Grade 36, Grade 55 can be supplied in a weldable version when supplementary requirement S1 is specified.
  • Grade 105: Offers high tensile strength and load-bearing capacity for critical structural connections.
  • A276 Stainless Steel and Duplex: These high-alloy materials offer corrosion resistance, with suitable grades providing enhanced protection against pitting and crevice corrosion in chloride-rich marine environments.

Fasteners Commonly Supplied With Anchor Bolts:

Anchor bolts are commonly supplied with complete fastening sets to ensure proper installation, alignment, and long-term performance. Here are the most common components supplied with anchor bolts:

  • Nut: Heavy hex nuts are commonly used with ASTM F1554 anchor bolt sets, with double nuts added where a jam nut is specified.
  • Washer: Sized to match the bolt hole, a plate washer can be used to cover oversized holes where required by the design.
  • Base Plate: The plate thickness and hole size are determined by the structural design requirements.
  • Template: Holds anchor bolt spacing and projection in position during concrete pouring. Misaligned bolts remain a common site issue, and using a template is a cost-effective way to prevent alignment problems.

Coatings and Finishes

  • Hot-Dip Galvanizing: Deposits a thick zinc layer to protect anchor bolts in outdoor and coastal environments. The nuts are tapped oversize where required to accommodate the zinc buildup.
  • Zinc Plating: Provides a thinner protective coating, generally suitable for dry indoor environments with limited exposure to moisture.
  • Black Finish: Refers to a thin conversion coating, such as black oxide, or an oil-treated mill finish applied to ferrous steel. Additional corrosion protection may be necessary depending on the service environment.

Frequently Asked Questions

Anchor bolt diameters generally span from M12 up to M64. That said, everyday structural applications lean far more heavily on the M16 to M36 range, especially for standard building construction and equipment baseplate installations.cify diameters between M16 and M36 for general construction and equipment baseplates.

Sum of structural embedment depth and exposed projection is total anchor bolt length. Required embedment length is determined by structural design and accounting for design loads. Moreover, concrete capacity, boundary conditions, and anchor spacing is also considered for accurate nominal values.

Yes. Hot-Dip Galvanizing applies a metallic zinc coating to the fastener surface which marginally increases its overall diameter. To compensate for this added thickness and ensure proper thread engagement, matching nuts are systematically tapped oversize.

Yes. The sizes listed in standard tables are simply typical figures. If your project demands specialized hook shapes, custom lengths, or non-standard heavy diameters, fabricators build them directly from structural shop drawings.

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