Socket head cap screw dimensions are shown in the following chart according to ASME B18.3-2003 standard.

Socket head cap screw is a type of cap screw with cylindrical head, smooth and knurled cylindrical sides, a flat bearing surface and hexagon or spline sockets for drive.

Some important notes from the ASME B18.3:

- The diameter range of socket head cap screws is from 0.06 inch to 4 inches according to standard.

- Thread shall conform to Unified external threads with radius root: Class 3A UNRC and UNRF for screw sizes No. 0 through 1 inch inclusive, Class 2A UNRC and UNRF for sizes over 1 inch through 1 1⁄2 inches inclusive, and Class 2A UNRC for larger sizes.

- Alloy steel cap screws shall conform to ASTM A547 and stainless steel cap screws shall conform to ASTM F837.


Nominal diameter (inches)
Nominal Length [L] (inches)1,2,3

Note1: Screws with nominal lengths less than the smallest length given in the dropdown list are full form (complete) threads. In this case, the threads extend to within two pitches of the head for sizes 0 (0.060 in) through 5/8 in inclusive; and shall extend as close to the head as is practice for sizes larger than 5/8 in.
Note2: For determination of min body length (LB), maximum grip gaging length (LG) and min thread length (LT) of screws with nominal lengths greater than the largest length given in the dropdown list, see ASME B18.3 standard.
Note3: Screws having nominal lengths falling between those for which given in the Nominal Length Dropdown List shall have LG and LB dimensions conforming with those of the next shorter nominal length for the respective screw size. For example: for a 1⁄4 in. size screw, 1.88 in. long, LG = 0.50 in. and LB= 0.25 in.


Dimensions of Socket Hex Cap Screws
Parameter Symbol Value Unit
Nominal Size --- --- ---
Body Diameter [D]1 Dmax --- in
Dmin ---
Head Diameter [A]2 Amax ---
Amin ---
Head Height [H] Hmax ---
Hmin ---
Max. Chamfer or Radius3 Cmax ---
Nom. Spline Socket Size M ---
Nom. Hexagon Socket Size J --- ---
Min. Key Engagement Tmin ---
Min. Wall Thickness Gmin ---
Max. Chamfer or Radius4 Kmax ---
Nom. Length L --- in 
Max. Grip Gaging Length LG_max ---
Min. Body Length LB_min ---
Min. Thread Length LT_min ---
Fillet Juncture Diameter
at Bearing Surface [F]
Fmax --- in
Fmin ---
Max. Fillet Length FL_max ---
Min. Juncture Radius Rmin ---
Before Threading Diameter
(In threaded to the head screws)
Nmin ---

Note1: Body refers to the unthreaded cylindrical portion of the shank for those screws not threaded to head.
Note2: Head may be plain or knurled at the option of the manufacturer.
Note3: The intersection of the top of the head and the side of the head may be chamfered or radiused within the limits of C, at the manufacturer's option.
Note4: The edge between the bearing surface and the side of the head may be broken, but the radius or chamfer shall not exceed Kmax.
Note:5 See the "Definitions" section for the details of the fillet.


Designation: Product Name, Designation of the Standard, Nominal Size - Thread Pitch x Nominal Length, Material, Protective Finish (if required) Ex: Hexagon Socket Head Cap Screws, ASME B18.3, 8 - 32 x 2.00, Alloy Steel, Zinc Plated

Fillet: For all lengths of screws, the form of the underhead fillet shall be optional, provided that it is smooth and continuous concave curve fairing into the bearing surface of the head, and the screw shank is within the envelope established by the limits for fillet extension, length, and juncture radius given in the results table.

Socket Head Cap Screw Underhead Fillets

Long Screw (Full Body Screw): Screw which is not threaded for full length.

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.

Short Screw (Thread to Head Screw): A screw which is threaded for full length.

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 Gaging : Specifications and dimensions for the gages used on Unified inch screw thread is 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.


Link Usage
Drill and Counterbore Sizes For Socket Head Cap Screws This calculator has been developed to calculate the recommended clearance hole sizes from #0 through 2 inches for socket head cap screws in two classes of clearance using close- and normal-fit category. The recommended inch drill bit size for each class of clearance hole is shown in calculation results.
Dimensions of Hex Keys (Allen Keys) and Bits Calculator covers inch series Hex Keys (Allen Keys) and Bits according to ASME B18.3-2003 standard.
Dimensions of Spline Keys and Bits Calculator covers inch series Spline Keys and Bits according to ASME B18.3-2003 standard.
Unified Inch Screw Thread Sizes & Tolerances Calculator Unified inch screw threads and tolerances calculator was developed to calculate basic major, minor and pitch diameters of the external (bolt) and internal (nut) inch thread according to ASME B1.1-2003 standard. In addition to basic size calculations, limits of sizes calculations of different tolerance classes can be done according to same standard.

  • Oberg.E , Jones.D.J., Holbrook L.H, Ryffel H.H., (2012) . Machinery's Handbook . 29th edition.  Industrial Press Inc. , pp 1682
  • ASME B18.3-2003, Socket Cap, Shoulder, Set Screws, Hex, and Spline Keys