Hex Bolt and Heavy Hex Bolt Dimensions according to ASME B18.2.1-2010 standard.

Hex bolt is a bolt with a hexagonal head shape and generally used in construction works of buildings, railways, bridges, factories, etc. The difference between hex bolt and heavy hex bolt is that heavy hex bolt has larger bolt head and head bearing area than hex bolt.

Some important notes from the ASME B18.2.1:

- The diameter range of hex bolts is from ¼ inch to 4 inches according to standard. For heavy hex bolts the range is from 3/8 inch to 3 inches.

- Bolt threads, when rolled shall be Unified inch coarse, fine or 8 thread series with tolerance class of 2A (UNRC, UNRF, or UNR-8). Threads that produced with other production methods may be Unified inch coarse, fine, or 8 thread series (UNC, UNF, or UN-8.)

- Unless otherwise specified, chemical and mechanical properties of steel hex bolts shall conform to ASTM A 307, Grade A. For heavy hex bolts, it shall conform to ASTM A 307, Grade B. Other materials and grades shall be as agreed upon by supplier and purchaser.

- Bolts with "reduced body diameter", the body diameter shall not be smaller than the minimum pitch diameter of the thread and not exceed the minimum body diameter given in the results table. To find the minimum pitch diameter of the thread, visit Unified Screw Thread Calculator.

- If either of the following conditions exists for the hex and heavy hex head bolt, the bolt shall be threaded for full length. For d ≤ 1 in, if L ≤ LT + 2.5*pitch; and for d > 1 in, if L ≤ LT + 3.5*pitch.

Nominal diameter (inches)
Bolt Length [L]
Bolt Type


Dimensions of Hex Bolts
Parameter Value
Product Name -- --- ---
Nominal Diameter d --- in
Full Size Body Diameter [E] Emax ---
Emin ---
Width Across Flats [F] Fbasic ---
Fmax ---
Fmin ---
Width Across Corners [G] Gmax  ---
Gmin ---
Head Height [H] Hbasic ---
Hmax ---
Hmin ---
Radius of Fillet [R] Rmax ---
Rmin ---
Thread Length LT_nom ---


Hex Cap Screw - Hex Bolt Differences:

- The bearing surface under the head of hex cap screw is flat and washer faced with thickness around 0.015 in to 0.035 in depends on the diameter of the screw. There is a tight run out tolerance for this face defined in ASME B18.2.1.

- Dimensional tolerance for body (or shank) diameter and head height is tighter in hex cap screws.

- There are different material options for hex bolt and hex cap screw: Hex bolts shall conform to ASTM A307, Grade A steel. For heavy hex bolts, it shall conform to ASTM A 307, Grade B. Hex cap screw shall conform to ASTM A307, ASTM A449, ASTM A354 GRADE BD or SAE J429.

 - It’s permissible to have die seam on the body of hex bolt within the specified tolerance values.

As a summary, hex cap screws are generally used in more precise applications due to better dimensional properties. Hex bolts are used in construction works and generally intended to be used with a nuts.

Unified Screw Thread: A thread form used by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. It has 60° thread angle and dimensions are stated in inch units.

UN form screw threads: The UN thread is intended for general purpose fastening operations.

UNC (Unified National Coarse): Most commonly used type for general engineering applications. This thread form is used in materials with low tensile strength which makes threads more resistance against stripping (internal threads) .UNC give possibility for quick assembly.

UNF (Unified National Fine): External threads of this Fine Series have greater tensile stress area than comparable sizes of the coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member. It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.

Thread class: A classification system to classify the threads for interchangeability and manufacturability. Class 1 threads are loosely fitting thread and used for ease of assembly. Most commonly used class is 2 and this class is designed for general use. Class 3 is used for closer tolerances.

Thread Gages and Gauging : Specifications and dimensions for the gages used on Unified inch screw thread are defined in the ANSI/ASME B1.2-1983 standard. Types of gages including thread plug gages, snap gages, thread ring gages and plain diameter gages for internal and external threads are covered in same standard. Screw thread gauging systems for acceptability are covered in the ASME B1.3-2007 standard.



  • Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2016) . Machinery's Handbook . 30th edition.  Industrial Press Inc.
  • Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2012) . Machinery's Handbook . 29th edition.  Industrial Press Inc.
  • ASME B18.2.1-2010, Square and Hex Bolts and Screws (Inch Series)