How Do I Know What Size Allen Wrench I Need?

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An Allen wrench comes in many convenient sizes. Those many sizes can also be perplexing when you are in the middle of a build. There are a few ways to determine the type of Allen wrench you need for a project. Hex wrench sizes are determined by the diameter of the screw. This is what you'll need to pay special attention to when choosing an Allen wrench.

How Do I Know What Size Allen Wrench I Need?
Image Credit: Anton Petrus/Moment/GettyImages

Allen Wrench Terminology

The Allen wrench dates back to the mid 1900s. It became the standard name for what had previously been called a hex key. The Allen Manufacturing Company created a set of quality hexagonal wrenches and produced them at a rapid pace.


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This made the former hex key be generally referred to as an Allen wrench, in the same way that brand names such as Kleenex and Band-Aid are used in place of the generic names of tissue and bandage.

The hexagonal wrenches are made in standard as well as metric sizes.


Standard-Size Allen Wrench

The inch-based set of Allen wrenches is considered standard in the industry. A typical set will contain a wide range of sizes, including:

  • 1/8 inch
  • 3/32 inch
  • 7/64 inch
  • 5/32 inch
  • 3/16 inch
  • 1/4 inch
  • 7/32 inch


European Standard-Size Allen Wrenches

The metric sizes are used as a standard in Europe. The typical sizes in the Allen wrench European set are:

  • 2.5 mm
  • 3 mm
  • 4 mm
  • 5 mm
  • 6 mm
  • 8 mm
  • 10 mm


If you are working on a project that has shipped from another country, you may need to use a metric set of Allen wrenches.

Which Size to Use

Unfortunately, screws with divots don't come with the size of Allen wrench they require stamped conveniently on the topside or side of the screw. This makes finding the correct-fitting Allen wrench a bit of a challenge.


If you truly aren't sure, you will have to make a good guess. To cut down on time wasted with trial and error, start with the middle wrench in the set and work your way up or down the sizes.

Allen Key Size Chart

Most Allen wrenches have the size stamped clearly on the side of the tiny, angled tool. The inch or millimeter size may have rubbed off with use or may be entirely too small to read. In this case, an Allen wrench or hex key chart can make finding the right size for your socket head much easier. Most manufacturers will publish an Allen key size chart.



An Allen key size chart often comes with furniture you need to build on your own, such as baby cribs, outdoor swings or computer desks. Hold on to it or post it somewhere in the garage where you can hold up the key or socket screw head to the physical picture. This can help you to determine which size of Allen wrench you need or have on hand.

Allen Wrench Use Tips

While Allen wrenches and hex keys seem durable enough, they can cause damage if you use the wrong size on a socket screw head. The smaller size of Allen wrench can easily strip a hex bolt or be stripped down itself.


Before placing the hex key into the bolt, make sure the head is clean of dirt and debris. A quick swipe with a Q-tip dipped in rubbing alcohol will pick up any dust that can collect in the corners of the hexagon.

Always make sure that each side of the Allen wrench is firmly and properly aligned before you begin to tighten or loosen a bolt. Otherwise, you can strip the bolt head or damage the wrench.



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