page_title: Hex key
section_title: Metric sizes
hierarchical_section_title: Hex key / Hex key standard sizes / Metric sizes
caption_attribution_description: English: Screw Size vs Allen Key Size for user
context_page_description: A hex key, Allen wrench or Allen key, is a simple tool used to drive bolts and screws with hexagonal sockets in their heads.
  The tool is usually formed of a single piece of hexagonal rod of hard steel, with blunt ends that are meant to fit snugly into the screw's socket, bent in an "L" shape with unequal arms. The tool is usually held and twisted by the long arm, creating a large torque at the tip of the short arm. Reversing the tool lets the long arm reach screws in hard-to-reach places.
  Each key is meant to be used with screws of a specific socket size, with rather tight tolerances; so the tool is commonly sold in kits that include half a dozen or more keys of different sizes. Usually the length of the key increases with the size of the socket, but not necessarily in direct proportion.
  Variants of the tool have the short end inserted in a transverse handle, which may contain multiple keys that can be folded into the handle when not in use.
  The "Allen" name is a registered trademark, originated by the Allen Manufacturing Company of Hartford, Connecticut circa 1910, and currently owned by Apex Tool Group, LLC.
context_section_description: Standard metric sizes are defined in ISO 2936:2014 "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
  24, 25, 27, 30, 32, 36, 42 and 46 mm.
  Metric hex wrench sizes are sometimes incorrectly 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 thread size of a metric screw or bolt.
  Metric hex keys are sorted in 1st choice sizes (which are most commonly used) and 2nd choice sizes (which are rarely used). A rationale for using only metric 1st choice sizes instead of a mix with 2nd choice sizes is that the 1st choice sizing intervals satisfy the constraint of being safe sizes. This means that it should not be possible to use a hex key with the wrong size, and thereby minimizing the risk of damaging either the screw head or the tool itself if the wrong tool should be used. A tool with a size too small will then simply spin freely inside the larger sized screw head. This is not true when mixing 1st and 2nd choice metric hex keys, or when mixing metric and inch-based hex keys, and as such these combinations do not constitute safe sizes.
  Table of some common metric hex key sizes from 0.7 to 10 mm. Italic values are rounded.
  1st choice column shows some of the sizes which are most commonly found on hex screws with ISO metric threads (M-threads)
  2nd choice sizes are rarely used.
caption: Screw Size vs Allen Key Sizes
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