Allen Key (Hex Key) and Hex Bit Sizes Chart to show dimensions of Allen keys (Hex keys) and bit sockets according to ASME B18.3-2003 standard.

Hexagonal (Hex) keys (Allen Keys) and bits sockets are tools which are used to drive screws with a hexagonal socket. Hexagon Keys are furnished as Short Arm Series or Long Arm Series.

Some important notes from the ASME B18.3:

- Hexagon keys and bits shall be fabricated from an alloy steel having two or more of the following alloying elements: chromium, nickel, molybdenum, or vanadium, in sufficient quantity to ensure that the specified minimum hardness of 48 HRC at the surface for nominal sizes up to and including 3⁄8 in., and 45 HRC for nominal sizes over 3⁄8 in., is met when hexagon keys and bits are hardened by quenching from the austenitizing temperature and tempered.

Allen Key (Hex Key) and Hex Bit Sizes Chart:

Nominal key or bit and socket size


Dimensions of Hex Keys and Bits
Parameter Value
Nominal Key Size (Frac.) --- --- ---
Nominal Key Size (Dec.) --- in
Hexagon Width
Across Flats [W]
Wmax ---
Wmin ---
Hexagon Width
 Across Corners [Y]
Ymax ---
Ymin ---
Length [B] Bmax ---
Bmin ---
Length [C]
(Short Series)
Cmax ---
Cmin ---
Length [C]
(Long Series)
Cmax ---
Cmin ---
Minimum Bend Radius Rmin ---
Bit Length 2,3 L ---
Maximum Chamfer Kmax ---

Note1: For plated hexagon keys and bits, all dimensions are before plating.

Note2: For nominal socket sizes above 1 in., it is recommended that bits be used in conjunction with standard hexagon wrenches or power drives. Bits 1 in. and smaller are available, but the lengths have not been standardized.

Note3: Length tolerance : +/- 0.062 inch


Designation: Product Name, Designation of the Standard, Nominal Key or Bit Size, Series, Protective Finish (if required) Ex: ASME B18.3, 1⁄8 Short Arm Series

Wrench: A mechanical hand tool which is used to hold, twist, or turn an object (such as a bolt or nut).


Link Usage
Hex Socket Dimensions Shows dimensions of a selected ASME(inch series) hex socket dimensions.
Socket Head Cap Screws Dimensions Shows dimensions of a selected ASME (inch series) hexagon and spline socket head cap screw.
Button Head Socket Cap Screws Dimensions Shows dimensions of a selected ASME (inch series) hexagon and spline socket button head cap screw.


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