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

Flat head socket cap screw is a type of cap screw with a flat top surface and a conical bearing surface with a head angle of approximately 82 deg. Both hexagon and spline socket types are available for drive.

Some important notes from the ASME B18.3:

- The diameter range of flat head socket cap screws is from 0.06 inch to 1.5 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.

- Alloy steel cap screws shall conform to ASTM F835 and stainless steel cap screws shall conform to ASTM F879.


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 shall extend to within two pitches of the intersection of the conical portion of the head with the basic screw diameter.
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, 2.13 in. long, LG = 0.75 in. and LB= 0.50 in.


Socket Flat Countersunk Head Cap Screws Dimensions
Parameter Symbol Value Unit
Nominal Size --- --- ---
Body Diameter [D]1 Dmax --- in
Dmin ---
Head Diameter [A]2 Theo.
Sharp Max.
Abs. Min. ---
Head Height [H] Href ---
Protrusion Gage Dia. [G] Gmax ---
Gmin ---
Protrusion [P]3 Pmax ---
Pmin ---
Spline Socket Size M ---
Nom. Hexagon Socket Size J --- ---
Min. Key Engagement Tmin ---
Fillet Transition Dia.4 Fmax ---
Nom. Length L --- in 
Max. Grip Gaging Length LG_max ---
Min. Body Length LB_min ---
Min. Thread Length LT_min ---

Note1: Body refers to the unthreaded cylindrical portion of the shank for those screws not threaded to head.
Note2: Maximum sharp value is a theoretical value and not practical to make the edges of the head sharp. This value represents the exact dia. of a hole countersunk to 82 deg, in which a screw having max. head size will fit flush.
Note3: The protrusion limits shown shall apply only when the gaging diameter is exactly indicated. See appendix of ASME B18.3 for details.
Note:4 A fillet between the conical bearing surface of the head and the shank of the screw is allowable above the max. F value.


Designation: Product Name, Designation of the Standard, Nominal Size - Thread Pitch x Nominal Length, Material, Protective Finish (if required) Ex: Hexagon Socket Flat Countersunk Head Cap Screws, ASME B18.3, 0.250 - 28 x 1.750, Stainless Steel

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
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 1684
  • ASME B18.3-2003, Socket Cap, Shoulder, Set Screws, Hex, and Spline Keys