Machine Screw Nut Sizes and Dimensions Chart according to ASME B18.6.3-2003 standard.

Machine screw hex nut is a hexagonal shape fastener which is used with a machine screw to fix parts together. 

Some important notes from the ASME B18.6.3:

- The diameter range of machine screw nuts is from 0.0600 inch to 0.3750 inch according to standard.

- Threads shall be Unified inch coarse or fine with tolerance class of 2B (UNC or UNF).

- Machine screw nuts are generally supplied in steel, stainless steel, or brass as specified by the purchaser. Unless otherwise specified, no physical requirements shall apply

Machine Screw Nut Sizes and Dimensions Chart:

Nominal size (inches)


Dimensions of Hex Machine Screw Nut
Parameter Value (inch)
Product Name -- ---
Nominal Diameter d ---
Width Across Flats [F] Fbasic ---
Fmax ---
Fmin ---
Width Across Corners [G] Gmax  ---
Gmin ---
Thickness Hex Nuts [H] Hmax ---
Hmin ---

Note 1: Hexagon machine screw nuts shall have top and bottom surfaces flat, with chamfered corners. The flat surface diameters shall be equal to the maximum width across flats, with a tolerance of minus 15%. The bearing surface shall be perpendicular to the axis of the threaded hole pitch cylinder within a tolerance of 4 deg.

Note 2: Bottoms of hexagon machine screw nuts may be supplied with flat, but unchamfered surfaces at the option of the manufacturer.


Gages and Gauging : 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. Screw thread gauging systems for acceptability is covered in the ASME B1.3-2007 standard.

Details of gages and gauging about the machine screws heads are given in the appendices of the ASME B18.6.3-2003 standard.

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.


Link Usage
Unified Inch Screw Thread Sizes & Tolerances Calculator Calculates 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 ,F.D. , Horton H.L. , Ryffel H.H., (2016) . Machinery's Handbook . 30th edition.  Industrial Press Inc. , pp 1693
  • ASME B18.6.3 - 2003, Machine Screws and Machine Screw Nuts