Standard U.S. Inch Bolt Size Chart

Not every diameter is offered in every thread series, grade, bolt type, or length; commercial availability varies by manufacturer and product specification.

US Bolt Size Notation

1/2-13 x 3 means a 1/2-in. nominal diameter, 13 TPI, and a 3-in. length. 3/4-10 UNC x 4 means a 3/4-in. diameter, 10 TPI coarse thread, and a 4-in. nominal bolt length.

An inch-series bolt size is primarily defined by nominal thread diameter, thread pitch or TPI, length, thread series and class, bolt type and head style, and grade or specification. All of these layers combine to fully describe a specific bolt; diameter by itself only identifies one part of that description.

SAE J429 currently covers mechanical and material requirements for inch-series steel bolts, screws, studs, sems, and U-bolts through 1-1/2 in. nominal size. UNC/UNF defines the thread system; SAE grade defines mechanical properties. These are two separate layers of specification, not synonyms.

3/8-16 x 2

Diameter = 3/8 in., TPI = 16, length = 2 in.

1/2-20 x 3-1/2

Diameter = 1/2 in., TPI = 20 (fine thread), length = 3-1/2 in.

1-1/4-8UN

Diameter = 1-1/4 in., using the separate 8-thread series rather than UNC or UNF.

Full dimension anatomy also includes major thread diameter, pitch diameter, minor diameter, head width across flats, and head height.

Product standards do not necessarily manufacture every fractional increment; this list reflects the commonly recognized commercial series.

UNC (Unified National Coarse) is the most common general-purpose thread series because coarse threads are relatively tolerant of dirt and damage and assemble quickly. For the complete UNC dimensional reference, see the UNC Thread Chart.

Fine threads use more threads per inch and have different tensile stress areas and thread engagement characteristics than UNC. For the complete UNF dimensional reference, see the UNF Thread Chart.

The 8-thread series (8UN) is used for several ASTM fastener specifications for diameters 1 in. and above, including some A193 and A320 applications. Do not assume every bolt 1 in. or larger uses 8UN; UNC and UNF remain common at those diameters too, depending on the product and specification.

Pitch equals 1 divided by TPI. For 1/2-13, pitch = 1/13 = 0.0769 in. For 1/2-20, pitch = 1/20 = 0.050 in. Common external thread classes include 1A, 2A, and 3A, with matching internal classes 1B, 2B, and 3B where applicable, all controlled under ASME B1.1.

ASME B1.1-2024 specifies thread form, thread series, class, allowance, tolerances, and thread designation for Unified inch threads, including UN, UNR, and UNJ thread forms, along with the UNC/UNF series context used throughout this page. Detailed thread geometry lives on the dedicated Thread Pitch Chart, UNC Thread Chart, and UNF Thread Chart.

For the complete dimensional reference, see the Hex Bolt Size Chart.

A 3/4-in. standard hex bolt has a shank and thread diameter of 3/4 in., but its head across flats is 1-1/8 in., which is also the correct socket or wrench size. For the complete socket reference, see the Socket Size Chart.

Standard hex bolts are used for general-purpose construction and mechanical fastening. Heavy hex bolts have a larger head across flats and greater bearing area. Heavy-hex structural bolt dimensional requirements are separated into ASME B18.2.6-2019, rather than being treated as ordinary B18.2.1 hex bolts.

ASME B18.2.1 covers complete general and dimensional information for multiple inch-series square and hex bolt and screw product types, including body diameter, head width, across corners, head height, and thread-length conventions. Heavy-hex structural bolts are specifically moved to ASME B18.2.6.

Commercially available lengths vary substantially by diameter, finish, and specification; this table shows general categories rather than one universal stock length series.

For ordinary hex-head bolts, measure from the bearing surface beneath the head to the end of the bolt. Countersunk fasteners can be measured differently because their heads sit within the material, typically measured from the top of the flush head instead.

These formulas are not universal for every bolt family. Carriage bolts, structural bolts, anchor bolts, and specialty products can use different thread-length requirements; structural A325/A490 bolts, for example, follow their own thread-length table rather than this general hex-bolt formula.

Do not make structural strength recommendations from thread location alone; this is a specification and ordering detail, not a substitute for engineering analysis.

The same nominal diameter does not mean the same head, length convention, thread length, or applicable standard across these types.

ASME B18.5-2012 (R2023) covers round-head inch-series bolt forms, including round-head square-neck carriage-type fasteners and other round-head products. Relevant dimensions include nominal diameter, thread pitch, head diameter, square neck width, and head height.

The current ASTM F3125/F3125M covers high-strength structural bolts and assemblies, including familiar A325/A490 designations and styles, paired with ASME B18.2.6-2019 for structural fastener dimensions, which covers heavy-hex structural bolts, compatible nuts, hardened washers, direct-tension indicators, and twist-off structural bolts. Full strength data lives on the Bolt Grade Chart.

The active ASTM F1554-20 covers straight, bent, headed, and headless anchor bolts and rods with grades 36, 55, and 105 for anchoring structural supports to concrete foundations. Anchor-bolt sizing involves diameter, overall length, thread length, bend or hook where applicable, embedment, and grade. For complete sizing and design data, see the Anchor Bolt Size Chart.

ASTM A307 covers carbon steel bolts, studs, and threaded rod with 60 ksi minimum tensile-strength requirements over a broad 1/4 through 4-in. size range. Bolt size and bolt material specification are separate concepts; A307 defines material and strength requirements, not the thread or dimensional system covered elsewhere on this page.

SAE J429 covers mechanical and material requirements for inch-series externally threaded steel fasteners through 1-1/2 in. Full proof load, tensile strength, and marking data are kept on the dedicated Bolt Grade Chart rather than duplicated here.

A 1/2-13 x 3 bolt can potentially be produced in different material or strength specifications. Diameter, pitch, and length describe size; grade or specification describes material and mechanical requirements. This prevents users from assuming “Grade 8” means a size rather than a strength classification.

For a 3/4-in. bolt, the mating nut thread is 3/4 in. with matching pitch, and the nominal washer is labeled 3/4 in., but actual nut and washer outside dimensions are larger and different from 3/4 in. Nominal fastener size keeps the assembly family together, but physical component dimensions are not all 3/4 in. See the Nut Size Chart and Washer Size Chart for the complete mating dimensions.