How Do I Determine My Hex Key Size?

Hex keys, also known as Allen wrenches, come in various sizes to fit different fasteners. Among the common metric hex key sizes, you can find options ranging from 1.5 mm to 10 mm. These sizes cater to a wide range of applications, from intricate tasks requiring precise 1.5 mm or 2 mm keys to heavier-duty work with 8 mm or 10 mm keys. Notably, some of the more frequently used sizes include 2.5 mm, 3 mm, 4 mm, 5 mm, 6 mm, 8 mm, and 10 mm. Whether you’re tackling a DIY project or professional-grade tasks, having the right size hex key on hand is crucial for achieving accurate and secure fastening. This information was last updated on February 25, 2020.

A size 4 hex key, also known as an Allen wrench, is a tool used to tighten or loosen screws and bolts with hexagonal sockets. Hex keys come in different measurement systems, including metric and imperial (SAE). In the metric system, a size 4 hex key is equivalent to 3 millimeters. In the imperial system, it is similar to 1/8 inch or 0.125 inches. This versatile tool is essential for various applications where hexagonal fasteners are commonly used. Whether you’re assembling furniture, working on machinery, or engaging in DIY projects, having a size 4 hex key in your toolkit can be quite handy.

“What is the physical dimension of a 4mm Allen key? This common tool, known for its hexagonal shape, plays a crucial role in various mechanical and DIY tasks. The ‘4mm’ measurement refers to the width across the flats of the hexagon, indicating the size of the tool. In essence, a 4mm Allen key has a hexagonal head with a distance of 4mm between opposite flat sides. This standardized sizing ensures compatibility with bolts, screws, and other fasteners requiring a 4mm Allen key for tightening or loosening.”

MM Substitutes for SAE

Metric Hex Key	Imperial Hex Size
3 mm	1/8
3.5 mm	9/64
4 mm	5/32
5.5 mm	7/32