Allen Wrench Sizes: Everything You Need to Know

Allen keys on a white background. — Most at-home assembly furniture toolkits come with allen wrenches because of how cheap they are. Elizabeth Fernandez / Getty Images

Allen wrenches go by a variety of names (allen keys, hex keys, etc.), and come in a wide variety of sizes and shapes. They come packaged with just about anything that requires assembly too. But using them effectively requires a bit of knowledge about allen wrench sizes.

What Are Allen Wrenches?

Named after the Allen Manufacturing Company that invented it, allen wrenches are "L" shaped tools made of alloy steel designed to work in conjunction with hex screws [source: Alloy Artifacts]. The tool is easy to use and incredibly small, which is why so many furniture manufacturers choose to include them in their packaging.

They're made up of a long arm and a short arm, both of which share the same key sizes. Allen wrenches have all sorts of practical applications outside of product assembly, which is why toolboxes aren't truly complete without one or two allen wrench sets, to ensure you're covered for both SAE and metric sizes.

Allen Wrench Sizes

You'll want a range of allen wrenches to make sure you have the right size for every particular need. The good news is that just about every tool manufacturer has a range of hex key sets you can purchase.

You can find a chart of the allen wrench sizes from any manufacturer [source: Truini]. The basic range of sizes (although not all sizes are included in all sets) is:

0.7 mm
0.9 mm
1.2 mm (3/64 inch)
1.3 mm
1.5 mm
1.6 mm (1/16 inch)
2.0 mm (5/64 inch)
2.4 mm (3/32 inch)
2.5 mm
2.8 mm (7/64 inch)
3.0 mm
3.2 mm (1/8 inch)
3.5 mm (9/64 inch)
4.0 mm (5/32 inch)
4.4 mm (11/64 inch)
4.5 mm
4.8 mm (3/16 inch)
5.0 mm
5.2 mm (13/64 inch)
5.5 mm (7/32 inch)
6.0 mm (15/64 inch)
6.4 mm (1/4 inch)
7.0 mm
8.0 mm (5/16 inch)
9.0 mm
9.5 mm (3/8 inch)
10.0 mm
12.7 mm (1/2 inch)

Using Allen Keys Effectively

The smaller-sized allen wrenches need more maintenance than the larger ones. You can more easily strip the head on a small allen key if you're not careful. So be sure that the hex bolt is clean before you insert the allen wrench and that you've inserted it with all six sides properly aligned [source: Morgan].

Also, if you buy a cheaper set of allen wrenches, the small ones may not be so precise in their measurements, which means they won't work as well (or eventually at all) within their size bolts. Substituting sizes is never recommended, as doing so can lead to a stripped screw head.