Hex keys standard sizes

Hex keys are measured across-flats (AF), which is the distance between two opposite (parallel) flat sides of the key. In some regions, hex keys are measured stochastically.

American sizes are defined in ANSI/ ASME standard B18.3-1998 "Socket Cap, Shoulder, and Set Screws (Inch Series)". Values given here are taken from Machinery's Handbook, 26th Edition, section "Fasteners", chapter "Cap and Set Screws", table 4 (p. 1601).

Metric hex wrench sizes are sometimes referred to using the designation "M" followed by the size in millimeters of the tool or socket, e.g. "M6", although this may be confused with the standard use of "M6" which refers to the threads of a metric screw or bolt.

Standard metric sizes are defined in ISO 2936:2001 "Assembly tools for screws and nuts—Hexagon socket screw keys", also known as DIN 911, and, measured in millimeters (mm) are 0.7, 0.9, 1.0, 1.25, 1.3, 1.5, 2 to 6 in 0.5 mm increments, 7 to 22 in 1 mm increments, followed by 24, 25, 27, 30, 32, 36, 42 and 46 mm.