U.S. Bolt Diameter Chart

Body tolerance depends on the actual bolt product and standard; this column reflects the ASME B18.2.1 standard hex bolt example, not every bolt type.

For threaded bolts, bolt diameter corresponds broadly to the major thread diameter. A 1/2-13 bolt has a nominal major diameter of 0.500 in., even though actual manufactured thread and body measurements are subject to tolerance.

Best tools are a digital caliper or a micrometer. Avoid using head width, wrench size, or thread root diameter to identify nominal bolt diameter.

Product standards do not necessarily manufacture every fractional increment; this list reflects recognized commercial series rather than every possible size.

A measured value should be interpreted with tolerance and thread condition in mind; a reading of 0.515 in. on a bolt is still consistent with a nominal 1/2-in. size under ASME B18.2.1 body tolerances.

Do not imply 1/2 in. equals M12 simply because 1/2 in. equals 12.7 mm, while M12 is nominally 12 mm; these are two different, non-interchangeable standards.

Metric commercial fastener sizes are selected separately under ISO 262, which covers ISO metric screw-thread sizes from 1 mm through 100 mm. Nearest size does not mean interchangeable.

Inch diameter uses fractions, metric uses nominal millimeters, inch pitch often uses TPI, and metric pitch uses mm per thread. See the US Bolt Sizes Chart and Metric Thread Chart for the complete systems.

A 1/2-in. diameter can use 1/2-13 UNC or 1/2-20 UNF. The diameter remains 0.500 in. in both cases, but thread spacing changes.

For full thread geometry, see the UNC Thread Chart.

For full thread geometry, see the UNF Thread Chart.

For larger industrial bolts, 8UN uses 8 threads per inch for selected diameters and specifications above 1 in. The 8-thread series is used in several ASTM fastener specifications for diameters 1 in. and above. Do not assume every bolt above 1 in. is 8UN; UNC and UNF remain common at those diameters too.

This table reflects the ASME B18.2.1 standard hex bolt example, not every bolt product; body tolerance rules vary by product and standard.

Not every bolt has an unthreaded body exactly equal to nominal thread diameter. Potential configurations include full body, reduced body, and fully threaded body. Do not infer structural performance from body appearance alone.

A 3/4-in. standard hex bolt has a nominal diameter of 3/4 in. and a standard head across flats of 1-1/8 in. Bolt diameter and head diameter or width are different dimensions. For the complete head dimensional reference, see the Hex Bolt Size Chart.

A 3/4-in. standard hex bolt has D = 3/4 in. but a wrench/socket size of 1-1/8 in. For a 3/4-in. heavy-hex bolt, the wrench/socket size is 1-1/4 in. See the Socket Size Chart for the complete reference.

A 3/4-in. standard hex bolt and a 3/4-in. heavy-hex bolt both have 3/4-in. nominal thread diameter, but their heads use different wrench sizes.

Full grade and strength data are kept on the dedicated Bolt Grade Chart.

Anchor rods and bolts are commonly specified primarily by diameter, length, grade, and embedment configuration, but diameter selection is a structural-design question, not a simple lookup. See the Anchor Bolt Size Chart for the complete sizing and design data.

Current ASME-based hex-lag examples include diameters from around 1/2 in. upward, with diameter-specific coarse wood-screw-style thread pitches rather than UNC threads. A lag screw does not use the same thread system as a machine or hex bolt just because nominal diameter is the same. See the Lag Bolt Sizes Chart for complete lag-specific data.

Carriage bolts are identified by nominal threaded diameter, along with a square neck and round head as distinguishing head features. Square-head bolts use the same nominal body-diameter convention as other inch-series bolts, but with different head geometry entirely. Do not make head diameter part of the main bolt-diameter table for these types.

A 3/4-in. bolt could potentially exist in different SAE grades, ASTM specifications, and finishes. Diameter tells you size; grade or specification tells you mechanical and material requirements. See the Bolt Grade Chart for grade and strength data.

Bolt size is often written as D x L or D-TPI x L. For example, 3/4-10 x 4 means diameter = 3/4 in., 10 TPI, and length = 4 in. The 4-in. dimension is not the diameter.

This page does not publish one universal hole-offset formula, because requirements vary by connection type, structural standard, fit class, and bolt diameter. Confirm the applicable hole size against the governing connection specification.

For a nominal 3/4-in. bolt, the nominal nut thread is 3/4 in. with matching pitch, and the nominal washer size is 3/4 in., but actual washer ID is greater than 3/4 in., and nut outside width is greater than 3/4 in. See the Nut Size Chart and Washer Size Chart for complete mating dimensions.

Rust, plating, damaged crests, and deformation can distort caliper readings. Measure several locations, use undamaged threads where possible, and combine diameter measurement with thread pitch rather than guessing from wrench size alone.