Allen Key (Hex Key) and Hex Bit Sizes Calculator was developed 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.


Nominal key or bit
and socket size


Dimensions of Hex Keys and Bits
Parameter Symbol Value Unit
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).

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