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How Do You Find the Hex Key Size?

A collage of different types of hex screw sizes.

Whether you call it a hex key, hex wrench, Allen wrench, or Allen key, this tool can be a handy little thing. Well, it can be when you can find the right size, anyway. When you are stuck trying to fit each one into the hex bolt head to find the right fit, it can actually be a bit frustrating.

Trial and error can also be a bad move since it can damage the hex screw or bolt, leaving you with no way to loosen or fasten it at all. Let’s prevent that by learning ways to find the exact size.

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Ways to Find the Hex Key Size

The following are the best methods for figuring out what size your hex wrench is.

Stay Organized

When you buy a hex key set, it usually comes in a packet or container that has each size neatly labeled for you. If you can put each one back after using them, you can always refer back to those labels.

These are stainless steel allen hex wrenches with labels for the various sizes.

Of course, this doesn’t help if you have little ones or others who like to borrow your tools. I’d say you could be like me and just keep an extra set tucked away that no one but you knows about. Somehow, though, no matter how hard I try, my “secret set” doesn’t remain secret for long.

Fortunately, there are a few other ways to determine the size.

Check Out the Tool Itself

Most hex keys come with the size imprinted on the side, which can be extremely helpful.

This is a close look at the embossed size labels on the allen wrenches.

Nothing’s perfect, though. Sometimes, those numbers are hard to see, especially if you already have vision problems. You could keep a magnifying glass around to help, but that’s another thing to keep up with.

And anyway, those numbers tend to get worn down after being used for a while.

You can always try going over it with a Sharpie or labeling them in some other way. For example, you might color code the sizes with permanent markers or nail polish and then make a color chart that you can tuck away in your toolbox.

Again, though, nothing’s perfect. It’s a good thing that there are a couple of other methods for when you can no longer see what’s on your tool.

Use a Hex Key Size Chart

There are nifty little charts available online to help, as well. The charts have hex key heads at their actual size. This means that you can put your key right up to the picture to figure out its size.

Consider printing out the chart, laminating it, and keeping it with your tools so that it’s always there when you need it.

Measure the Hex Key

As long as you know how to measure a hex key, you can always determine the size you have. Hex keys measurements are done across-flats or AF.

This simply means that they are measured from one flat side of the head to the one directly across from it. Thank goodness for this little bit of knowledge. It means that you can actually figure out the hex key size with your own measuring tape or digital caliper if you have one.

These are a couple of stainless steel allen hex key wrenches measured with  a ruler.

How Do I Know Which Size I Need for My Project?

If you bought a project-in-a-box, like a furniture you might buy at Target, Walmart, or Amazon, you are probably covered. Most of those sets come with the correct size hex key. If not, you can probably find a list of supplies you need in the instructions.

We all know, though, that these boxes can sometimes be missing important things. If you do not have the instructions and there is no hex key included, it’s still not hopeless. You have two options:

  1. Measure the fastener hole the hex key is supposed to fit into.
  2. Do the old-school trial and error.

If you choose to go the trial and error method, there are a couple of important things to do. First, start with a mid-size hex key. This will give you a better idea of the size you need without having to try too many.

The correct size of Allen hex key wrench was used for the screw.

Second, do not use a lot of force to try it out. Doing this risks you warping the screw. Instead, hold the screw in your hand to try each size. When you find one that fits snugly, you have a winner.

Where to Buy a Hex Key Set

You can find a good hex key or Allen wrench set at most retailers, in-store and online. Some of the most common places to shop are:

  • Harbor Freight
  • Walmart
  • Amazon
  • Lowe’s
  • Home Depot

You are more likely to find a wide range of hex key sizes from hardware stores, like Lowe’s and Home Depot. Another great option is a tool manufacturer like Bondhus.

Still, you can find standard sets at most of the stores listed here. Where you should buy really depends on the range of sizes you need.

Frequently Asked Questions

What different sizes does an Allen key come in?

You can get two types of hex key sets: standard key size and metric key size. Standard typically comes in 1/8 inch, 3/32 inch, 7/64 inch, 5/32 inch, 3/16 inch, ¼ inch, and 7/32 inch. Metric sizes most often come as 2.5 mm, 3 mm, 4 mm, 5 mm, 6 mm, 8 mm, and 10 mm. Some sets come in many other sizes

Some sets come in half sizes as well. You should also keep in mind that standard hex keys come with a short arm. However, you can purchase long arm hex keys if you need some additional length.

Why are they called hex keys?

Hex keys have six sides, making them a hexagonal shape. That word was simply shortened to fit the tool.

Why do some people call them Allen wrenches?

Hex keys were originally produced by a company known as Allen Manufacturing, and so the tool carried the Allen name. When people refer to “Allen keys” and “Allen wrenches,” they are simply referring to the company that created the product. Hex keys are just the generic term for the brand name product.

What if I don’t have the exact size I need?

If you can’t find the hex wrench size you need in your hex key set, you have a couple of other things you can try. The first only helps if you have an interchangeable screwdriver — you know, the type that lets you change out the head. Sometimes, the head tips that come with them will include hex keys, so you might find what you need there.

The next option is to use a flathead screwdriver. This will only work if you have one whose tips fit into the corners of the hex bolt or screw. If it fits snugly, you can try to turn it with the screwdriver.

If these tricks don’t work, it’s better that you wait until you can buy or borrow the right size. It’s too risky for your project and tools to use the wrong size.

What if I already stripped an Allen screw?

Well, nobody’s perfect. I can’t tell you how many times I stripped some of my own and experienced the utter frustration of trying to remove the screw. Good news, though: there is a way to do it.

Try cutting a slot just big enough for a flathead screwdriver into the screw head. Place the screwdriver into the slot. With a hammer, tap the screwdriver in a counter-clockwise direction to loosen up the screw.