Nine inch-series diameters from 1/2 to 1-1/2 in. are standard. For general inch bolt dimensions, see the US Bolt Sizes Chart. For the full family of structural bolt grades, see the Structural Bolt Size Chart.

The heavy-hex head is 1/8 in. wider across flats than a regular hex head of the same diameter, and the structural thread length is shorter than a general-purpose bolt, so the unthreaded shank stays in the shear plane. See the thread length section below.

An ASTM A325 structural bolt is a high-strength heavy-hex bolt for steel-to-steel connections. It is made from quenched and tempered medium-carbon or alloy steel, supplied with matching heavy-hex nuts and hardened washers, and installed by a controlled method under the RCSC specification. The grade is marked on the head.

A325 bolts connect beams, columns, braces, gussets and splice plates in buildings, industrial structures and bridges. They are used in bearing-type joints and in pretensioned or slip-critical joints.

The wider head gives a larger bearing surface for the clamping load. Do not confuse an A325 bolt with an ordinary hex cap screw, whose head and thread length differ, or with a foundation anchor rod, which anchors steel to concrete. See the Anchor Bolt Grade Chart and Anchor Bolt Size Chart for anchor rods.

The standalone ASTM A325 specification was withdrawn in 2016 and consolidated into ASTM F3125/F3125M. The familiar “A325” designation survives as Grade A325 inside F3125, so the correct current wording is “ASTM F3125 Grade A325.” The current edition is F3125/F3125M-26, which ASTM lists as active as of July 31, 2026. Your project documents may reference an earlier edition, so always follow the edition adopted by the contract.

Before consolidation, A325 bolts above 1 in. had lower minimum strengths than smaller sizes. Old drawings and old spreadsheets may still reflect that.

F3125 sets a uniform 120 ksi minimum tensile strength for Grade A325 in all sizes from 1/2 to 1-1/2 in. The inch and SI products are separate, and direct conversion between them is discouraged by RCSC.

The full F3125 text governs chemistry, mechanical properties, manufacture and marking. It is paid content, so use the ASTM document, or the manufacturer’s certification, when a requirement matters.

The nine standard inch diameters run from 1/2 to 1-1/2 in. They are specification sizes, not a promise that every length and finish is stocked by every supplier. Converting an inch diameter to millimeters does not make it an interchangeable metric product.

Nominal diameter is a designation. A real bolt body is measured within the tolerance set by ASME B18.2.6, and the maximum body diameter in the table is slightly larger than the nominal size.

ASME B18.2.6 gives nominal values and tolerances for the heavy-hex head. A 3/4 in. structural bolt has a nominal head width of 1-1/4 in., so the wrench or socket is larger than the shank. Heavy hex differs from standard hex because the head is wider across flats than a standard hex head of the same bolt diameter.

This is the wrench size. For a 1 in. A325 bolt, the head is 1-5/8 in. across flats, so a 1-5/8 in. socket or wrench fits. See the US Bolt Sizes Chart for general bolts.

The corner-to-corner width matters for tool clearance and is larger than the across-flats value.

Head height is a nominal reference, and actual heads fall between the minimum and maximum values.

A325 structural bolts use the Unified National Coarse (UNC) thread series unless the order says otherwise. The number after the hyphen is threads per inch (TPI).

Structural bolts have shorter thread lengths than general-purpose heavy-hex bolts. The standard thread length is the nominal length of thread measured from the end of the bolt, and it keeps the unthreaded shank in the shear plane when the grip is controlled. Thread length and overall bolt length are different dimensions.

For a 3/4 in. A325 bolt the nominal thread length is 1-3/8 in. The bolt can be any of many lengths, so a 3/4 × 3-1/4 bolt has 1-3/8 in. of thread and the rest is unthreaded body.

F3125 permits a fully threaded A325 bolt under a supplement, limited to lengths up to four bolt diameters, and the bolt is marked A325T. A “S” marking identifies altered thread length or head dimensions that still comply, and an “M” marking denotes the metric version. Special threading configurations must be identified separately and are not the standard structural bolt.

Between the full unthreaded body and the full threads is a transition. Whether the shear plane passes through threads (N) or the plain body (X) changes shear strength, so the thread position matters when choosing length.

Required bolt length is the grip plus the thickness of any washers plus a tabulated allowance for the nut and thread projection. Nominal bolt length is measured under the head, so it excludes head height.

Measure under the head to the end of the bolt. Structural bolts can be made shorter than nominal by the amounts below, so allow for it when selecting.

Type 1 and Type 3 are material classifications. They share the same heavy-hex dimensions, thread system and Group 120 strength level.

Type 3 is neither stainless steel nor simply galvanized Type 1 steel. It suits outdoor exposure with alternating wet and dry conditions, and it may not form a stable patina in continuously damp or marine environments. Matching nuts and washers are Type 3 as well.

F3125 sets the mechanical requirements for Grade A325. The values below come from the tension and hardness tables of the specification text reviewed.

RCSC 2025 Table 3.1 lists nominal bolt hole dimensions. Standard holes are 1/16 in. larger than the bolt through 7/8 in., and 1/8 in. larger from 1 in. up. Oversize holes and slots follow the formulas shown for 1-1/8 in. and larger bolts. Do not confuse these with concrete anchor drill-bit sizes or general machine-bolt clearance holes. For those, see the Bolt Hole Size Chart and Clearance Hole Size Chart.

The default hole. The nominal diametral difference for a 3/4 in. bolt is 1/16 in., which is not the radial clearance on each side. The ideal radial clearance is 1/32 in.

Larger round holes used only where the joint type and design permit. They are not allowed in every connection.

Slots allow adjustment. The slot direction, which plies may be slotted, and the washer requirements depend on RCSC and the connection design.

Bolt diameter alone does not set the hole layout. AISC requires a minimum center-to-center spacing of 2⅔ bolt diameters and prefers 3d. Edge distance is measured from the center of a hole to the edge of the connected part, and end distance is the same measurement in the direction of force. Plate thickness, bearing and tearout also decide the layout.

For a 3/4 in. bolt the minimum is 2 in. and the preferred spacing is 2-1/4 in. For layout help see the Bolt Pattern Calculator.

Values depend on hole type and edge condition. Do not create a universal “1.5d” rule.

A correct bolt diameter does not guarantee adequate plate bearing, tearout or block shear, so those limit states must be checked. Plate dimensions are in the Steel Plate Thickness Chart.

A325 bolts use heavy-hex nuts made to ASTM A563, or ASTM A194 Grade 2H as a permitted alternative. RCSC 2025 also permits A194 DH-type nuts as a substitute for A563 Grades C, D and DH. A nut with the right thread size is not automatically a compliant structural nut, so it must be the specified grade and style for the bolt type and finish.

Heavy-hex nuts have the same width across flats as the heavy-hex bolt head. ASME B18.2.6 lists nominal nut thicknesses such as 39/64 in. for 5/8 in., 47/64 in. for 3/4 in. and 55/64 in. for 7/8 in. See the Nut Size Chart for general nut dimensions.

The nut thread must match the bolt UNC thread, and a galvanized bolt needs a nut and lubricant combination that suits the coating. Use matched assemblies.

ASTM F436/F436M hardened washers are used with A325 assemblies. Washer use and placement depend on the joint, the installation method, the hole type and the component configuration. RCSC Section 6 sets where washers are required, and the 2025 edition adds ASTM F3394 hardened backup and reaction washers for certain installation tools.

Washer placement can depend on the installation method, and plate washers or continuous bars are used for some slotted or oversized conditions. See the Washer Size Chart for other washer types.

Oversized and slotted holes can require washers or plate washers that snug-tightened standard-hole joints do not. Check RCSC for the exact requirement.

Genuine Grade A325 heavy-hex bolts carry specified head markings. These typically show the grade, the manufacturer identification and, for Type 3, an underlined grade mark.

• Grade marking: the grade designation on the head.
• Manufacturer identification: the maker mark.
• Type 1 and Type 3: Type 3 shows an underlined grade mark.
• Special configurations: T for fully threaded (up to four diameters), S for altered thread or head dimensions, M for metric.
• Documentation: coating and packaging information and mill or manufacturer test reports.