Fasteners are available in different types, sizes, and metals. Fasteners with a hexagonal-shaped recess in the heads of the fastener are among the most useful.
These fasteners are referred to as hexagonal socket head screws or Allen screws, after the inventor. The fasteners are meant to be placed and removed with the aid of a specialized tool referred to as an Allen wrench or Allen key.
Allen wrenches are one of the most popular and frequently used tools in the toolkit of the average do-it-yourselfer. Their L-shaped heads easily identify them.
They are used for various applications, but mostly in vehicle repair, unclogging garbage disposals in sinks, and assembling prefabricated furniture.
Allen wrenches are frequently relatively small and are available in various standard SAE International sizes, including 5/32-inch, 3/32-inch, 7/16-inch, 3/16-inch, 1/18-inch, and 14-inch.
In our guide, you can learn more about how to use our Allen head size chart. By the end, you’ll no longer be stuck on determining Allen wrench size for the screw, as you can see how to measure Allen wrench size, so you pick the correct one. (Read Sockets Sizes in Order)
How Do I Know My Allen Key Size?
The Allen wrench comes from the mid-1900s and has become the standard for what we know as a hex key. The Allen Manufacturing Company is a registered trademark and created a set of quality hexagonal wrenches, which are standard and metric sizes, where Allen key sizes in mm are common.
Here you can find the only hex key size chart and the Allen wrench size guide you’ll ever need. The hex wrench sizes chart cover metric, and the Allen key size chart metric, and an Allen wrench chart where the Allen keys are made from another material and have the ball head.
Allen Keys Metric
| Hex size | Long arm length | Short arm length |
|---|---|---|
| 1.5 mm | 90 mm | 14 mm |
| 2 mm | 112 mm | 19 mm |
| 2.5 mm | 101 mm | 16 mm |
| 3 mm | 123 mm | 21 mm |
| 4 mm | 137 mm | 24 mm |
| 5 mm | 154 mm | 27 mm |
| 6 mm | 172 mm | 31 mm |
| 8 mm | 195 mm | 37 mm |
| 10 mm | 224 mm | 42 mm |
Allen Keys Common Imperial Sizes
| Hex size | Long arm length | Short arm length |
|---|---|---|
| 3/32 inch | 112 mm | 19 mm |
| 7/64 inch | 119 mm | 20 mm |
| 1/8 inch | 123 mm | 21 mm |
| 9/64 inch | 130 mm | 22 mm |
| 5/32 inch | 137 mm | 24 mm |
| 3/16 inch | 154 mm | 27 mm |
| 1/4 inch | 185 mm | 34 mm |
| 5/16 inch | 195 mm | 37 mm |
| 3/8 inch | 224 mm | 42 mm |
Allen Keys Common SAE Sizes – Imperial Alternative Material and Ball End
| Hex size | Long arm length | Short arm length |
|---|---|---|
| 0.05 inch | 83 mm | 14 mm |
| 1/16 inch | 91 mm | 15 mm |
| 5/64 inch | 100 mm | 16 mm |
| 3/32 inch | 112 mm | 18 mm |
| 7/64 inch | 119 mm | 19 mm |
| 1/8 inch | 126 mm | 20 mm |
| 9/64 inch | 132 mm | 23 mm |
| 5/32 inch | 140 mm | 25 mm |
| 3/16 inch | 160 mm | 25 mm |
| 7/32 inch | 180 mm | 27 mm |
| 1/4 inch | 190 mm | 34 mm |
| 5/16 inch | 200 mm | 36 mm |
| 1/2 inch | 224 mm | 40 mm |
Which Size Do I Use
Allen screws are not pre-stamped on the top or side of the screw with the Allen wrench size needed and can add to the difficulty of locating the correct Allen wrench.
If you are uncertain, make a guess, and to minimize time wasted, you can start with the middle wrench in Allen wrench set sizes you have and work up or down the sizes until you find the right one.
The Allen wrench sizes are stamped on the side of the tool. The inch or millimeter size could wear, and it is here where an Allen wrench or hex key chart can significantly simplify determining the correct size for your socket head. (Read Star Bits Sizes Chart)
Most manufacturers include an Allen key size chart with their products.
An Allen key size chart is frequently included with self-assembled furniture, such as infant cribs, outdoor swings, or computer tables.
Hold onto your Allen key size chart in mm or metric, as using the wrong size Allen key on your hex bolt could easily strip the inside.
It is the manufacturer’s guide where you can learn how to measure Allen key size when the writing has gone. How to measure an Allen wrench is as easy as holding your hexagon Allen key or socket screw head against the diagram or image to determine the Allen wrench size.
What Size is a 4mm Allen Key?
Here you can find the best SAE Allen wrench for a given metric size. Metric to SAE hex wrenches don’t always have a perfect opposite, so you’ll need a separate metric hex key set, which is recommended as more hardware is produced internationally.
SAE and metric hex key sets can live together in your tool kit.
Last, having SAE & metric hex keys in the United States, you’ll find larger metric half sizes of sockets like 3.5 mm aren’t standard. However, a 9/64 Allen wrench in mm as an SAE hex wrench works in place of the 3.5 mm wrench. A 3/32 Allen wrench in mm, by comparison, would be 2mm.
Here, you can find the different sizes of the socket wrench and how it converts from metric to SAE. Between the two, you’ll find the right sizes Allen key for your screw.
Metric Hex Key SAE Hex Size
| Metric Hex Key | SAE Hex Size |
|---|---|
| 2 mm | 3/32 |
| 2.5 mm | 7/64 |
| 3 mm | 1/8 |
| 3.5 mm | 9/64 |
| 4 mm | 5/32 |
| 5.5 mm | 7/32 |
| 6 mm | 1/4 |
| 7 mm | 9/32 |
| 8 mm | 5/16 |
| 9 mm | 3/8 |
What is the Most Common Allen Key Size?
In standard measurements, the most common Allen wrench sizes and bolts for common tasks are 5/32-inch, 3/16-inch, and ¼-inch sizes.
What Size is a T40 Allen key?
A T40 would be 9/32″ or 7mm.
Metric Sizes or Standard Sizes?
Using your Allen wrenches can dictate if the sizes are standard or in Metric. The right size, for instance, could be a metric if you have a foreign car.
Types of Allen Wrenches
Allen wrenches or Allen keys are available in different styles. The most common is the L-shaped hex key, yet you have variations incorporating different handle designs, different materials, or special features.
T-shaped Allen keys – T style Allen wrenches differ from traditional Allen wrenches in that they have a handle created from a wrench in the metal stock that leads to a single drive key centered across the handle’s width.
It enables a more pleasant action when used repeatedly and allows for greater torque to be exerted than with the traditional L-shaped Allen key, which is also available. Short and long lengths are also offered, so you can insert the shorter tools into tighter spaces or corners where a longer tool wouldn’t fit in the bolt.
Ball-end Allen keys – The ball head Allen key is one of the more distinctive types of Allen Key available. They are distinguished by the presence of a hexagon-shaped ball at the driving end of the tool. Allen wrenches of this sort are available in two different types: L-shaped and T-shaped designs.
Unlike traditional straight profiles, the ball end design on the driver allows for the tightening or loosening of fasteners when the tool is held at an angle concerning the fastener, which is not possible with the straight profile.
This is particularly useful in situations where the working clearances are such that a head-on usage of the tool is impossible or where a direct engagement would not provide enough room for the tool to rotate to apply torque to the application. (Read LightSocket Sizes Chart)
In general, these wrenches are not advised for high torque situations since the engagement with the fastener when the ball end is used is diminished when using this form of the wrench.
Excessive torque might cause the material of the hex surfaces of the socket to become stripped or the inserted ball of the wrench to come loose. Long and short lengths are also available in a variety of colors.
Some ball-end Allen keys have a ball at one end of the driver and a shorter driver extending off the handle with a regular hex head at the other end of the driver. Others may choose to use a ball at both ends of the tool or a single driver with a handle on one end of it.
