Inch Hex Bolt Dimensions

This chart covers ordinary inch-series hex bolts. Heavy-hex structural bolts and metric hex bolts use separate standards and dimensions covered later on this page.

A hex bolt has a six-sided head, a cylindrical body or shank, and an external screw thread. Ordinary hex bolts are commonly used in construction, general fabrication, equipment, machinery, and supports and frames.

A hex bolt is typically tightened by turning the nut while the head is held stationary. A hex cap screw is commonly tightened by turning the head, often into a tapped hole, and cap screws generally have tighter dimensional tolerances than ordinary hex bolts.

Dimensional Differences

Why This Matters

ASME B18.2.1 applies different dimensional controls to the two product forms, so hex-cap-screw dimensions should not be mixed into the primary hex-bolt chart on this page.

Heavy-hex structural bolts formerly covered under B18.2.1 are now covered separately under ASME B18.2.6, and heavy-hex bolts use a larger head across flats than an ordinary hex bolt of the same nominal diameter. A 3/4-in. ordinary hex bolt and a 3/4-in. heavy-hex bolt do not necessarily use the same wrench size. For mechanical grade and material strength data, see the Bolt Grade Chart.

This designation means a nominal diameter of 1/2 in., a thread pitch of 13 threads per inch, and a nominal bolt length of 3 in.

Diameter

The first number identifies the nominal shank or thread diameter.

TPI

Threads per inch, the second number in the designation.

Length

Measured from the bearing surface under the head to the bolt end for ordinary hex-head bolts.

Key dimensions include nominal body diameter (D), width across flats (F), width across corners, head height (H), nominal bolt length (L), nominal thread length (LT), fillet radius, thread runout, and chamfer.

ASME B18.2.1 includes ordinary hex bolts across this broad inch-series size range and beyond; this chart follows the standard rather than an arbitrary “common size” cutoff.

Width across flats (F) is the distance measured between two parallel flat sides of the hex head.

This section directly supports wrench selection, since across-flats is the dimension a standard open-end or box wrench actually grips.

Across-corners matters for socket clearance, recessed installations, adjacent-part clearance, and CAD or layout work. ASME B18.2.1 provides both across-flat and across-corner limits for every standard diameter.

ASME B18.2.1 controls these dimensional limits, with maximum and minimum tolerances published alongside the basic value for each diameter.

ASME B18.2.1 permits dimensional tolerances on body diameter. A 1-in. hex bolt, for example, has a body diameter maximum of 1.022 in. under the standard. This is a useful practical reason clearance holes must not be sized from nominal diameter alone.

Measure from the bearing surface beneath the head to the end of the bolt. Actual production length has applicable manufacturing tolerances, so nominal length and precisely measured length can differ slightly. Selecting bolt length requires accounting for grip thickness, washer, nut, and required thread engagement or protrusion; there is no single universal “add X threads” rule that applies across every standard and application.

Bolt length and thread length are not the same thing, which is especially important to keep straight when specifying partially threaded bolts for a clamped joint.

These values are calculated directly from the standard formula and verified arithmetically. Do not apply this formula beyond its standard scope; bolts longer than 6 in. use a different allowance under ASME B18.2.1.

This distinction affects where the shear plane falls in a bolted connection and how much of the bolt engages threads versus smooth shank, which is relevant to grip and assembly. This page focuses on size identification, not full structural connection design.

Pitch P = 1/TPI, expressed in inches. For a full dimensional UNC reference including tap drill sizes, see the UNC Thread Chart.

Fine threads provide more threads per inch and a smaller pitch than coarse threads at the same diameter. For a full UNF reference, see the UNF Thread Chart.

Do not assume UNF is always stronger or UNC always better under vibration; application and failure mode determine which series is appropriate for a given joint.

Thread classes such as 1A, 2A, and 3A apply to external Unified threads, describing allowance and tolerance grade. ASME B1.1 controls thread form, series, class, allowance, tolerance, and designation rules for Unified inch threads. For deeper thread geometry and pitch data, see the Thread Pitch Chart.

Clearance-hole diameter depends on bolt diameter, required fit, applicable design or specification, and fabrication standard. This page does not use a universal “hole equals bolt plus 1/16 in.” rule, since actual clearance requirements vary by application and specification. For threaded-hole sizing specifically, see the Tap Drill Chart, noting that tap-drill size is for cutting internal threads and is not the same thing as a clearance-hole size for a through-bolt.

Nut compatibility requires matching nominal diameter, thread pitch and series, and any material or grade requirement. A 1/2-13 UNC bolt requires a compatible 1/2-13 nut, not just any 1/2-in. nut. Common nut types include finished hex nuts, heavy hex nuts, and locking nuts. Washer selection similarly depends on bolt diameter, washer specification, application, and bearing requirements; not every 3/4-in. bolt uses one universal washer outside diameter or thickness, and structural high-strength systems can have specific washer requirements of their own.

Common head markings can indicate mechanical grade or specification. SAE J429 covers mechanical and material requirements for inch-series externally threaded steel fasteners through 1-1/2 in. and includes familiar SAE bolt grades. For complete strength and marking data, see the Bolt Grade Chart, and for torque values by grade and diameter, see the Bolt Torque Chart.

SAE Grade 2, Grade 5, and Grade 8 Hex Fasteners

Familiar SAE grades affect material and proof or tensile strength and head markings, but they do not change nominal bolt diameter or thread designation. Full strength details are covered on the dedicated Bolt Grade Chart rather than duplicated here.

Possible specifications for hex bolts include ASTM A307, ASTM A354, ASTM A449, and other application-specific standards. “Hex bolt” by itself does not identify material grade; dimensions, threads, and mechanical specification are separate layers of the fastener designation and must each be specified independently.

Current ASTM F3125/F3125M covers high-strength structural bolts and assemblies, including A325 and A490 heavy-hex styles, with inch heavy-hex sizes generally from 1/2 in. through 1-1/2 in. depending on grade and style. A325/A490-style structural bolts should not be treated as ordinary hex bolts simply because both have six-sided heads; their head proportions, nut and washer systems, strength, installation, and structural design requirements are all different from a general-purpose hex bolt.

ISO 4014:2022 covers hexagon-head bolts with metric coarse-pitch threads M1.6 through M64, product grades A and B. Some diameters, such as M10 and M12, have a different wrench size under the older DIN 931/933 standard versus current ISO 4014, so the applicable standard must be confirmed before selecting a wrench. Do not simply convert inch hex dimensions into metric; metric fasteners have their own standardized dimensions. For full thread geometry, see the Metric Thread Chart.

Metric Hex Bolt vs Metric Hex Head Screw

ISO distinguishes ISO 4014:2022 (hexagon head bolts) from ISO 4017:2022 (hexagon head screws), which parallels the inch-series bolt versus cap-screw distinction covered earlier on this page.

Do not substitute an inch wrench or bolt combination merely because dimensions are numerically close to a metric equivalent; the thread forms are not interchangeable even when diameters appear similar.

ASME B18.2.1 covers general and dimensional data for inch-series square and hex bolts and screws. Heavy-hex structural bolts formerly covered under this standard are now addressed separately under ASME B18.2.6, which is why this page maintains a clear boundary between the two product categories throughout.

ASME B1.1-2024 is the current Unified Inch Thread standard covering UN, UNR, and UNJ thread forms and thread-series and designation rules. All UNC and UNF pitch data on this page should be understood as reflecting the current standard rather than an outdated thread table.