Hex Key Standard Sizes
Hex keys are measured across-flats (AF), which is the distance between two opposite (parallel) flat sides of the key. 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
- 24, 25, 27, 30, 32, 36, 42 and 46 mm.
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.
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).
| Screw size (nominal) | Socket size (inches) | Approximate socket size (mm) |
|---|---|---|
| No. 6 | 7/64 | 2.78 |
| No. 8 | 9/64 | 3.57 |
| No. 10 | 5/32 | 3.97 |
| 1/4 | 3/16 | 4.76 |
| 5/16 | 1/4 | 6.35 |
| 3/8 | 5/16 | 7.94 |
| 7/16 | 3/8 | 9.52 |
| 1/2 | 3/8 | 9.52 |
| 5/8 | 1/2 | 12.7 |
| 3/4 | 5/8 | 15.9 |
| 7/8 | 3/4 | 19.0 |
| 1 | 3/4 | 19.0 |
Note that numerous other sizes are defined; these are the most common.
Using a hex wrench on a socket that is too large may result in damage to the fastener or the tool. An example would be using a 5 mm tool in a 5.5 mm socket. Because hex-style hardware and tools are available in both metric and British/Imperial sizes (with British sizes most commonly used in the United States and denoted "SAE", and metric in other places), it is also possible to select a tool that is too small for the fastener by using a British-unit/Imperial tool on a metric fastener, or the converse. There are some exceptions to that. For example, 4 mm keys are almost exactly the same size as 5/32", and 8 mm keys are almost exactly the same size as 5/16", which makes 4 mm and 8 mm preferred numbers for consumer products such as self-assembly particle-board furniture, because end users can successfully use an imperial key on a metric fastener, or vice versa, without stripping. 19 mm keys are so close to the same size as 3/4" that they are completely interchangeable in practical use.
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