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SAE vs. Metric Hex Keys: Which Do You Need? by Hi-Spec

SAE vs. Metric Hex Keys: Which Do You Need?

What are hex keys?

Hex keys, also known as Allen keys or Allen wrenches, are small, L-shaped tools with a hexagonal cross-section used to drive bolts and screws with a hexagonal socket. These tools are available in various sizes and can be found in both metric and SAE (Society of Automotive Engineers) measurements.

Related Article: The Best DIY Hex Wrench Set

Types of hex keys

While the standard L-shaped hex key is the most common type, there are several other variations that offer unique benefits for specific applications. These include:

  • T-handle hex keys: These tools feature a T-shaped handle, providing increased leverage and torque compared to the standard L-shaped design. 
  • Folding hex key sets: Folding sets contain multiple hex key sizes in a compact, fold-out design resembling a Swiss Army knife. 
  • Ball-end hex keys: Ball-end hex keys have a rounded tip, which allows for better contact with the fastener at various angles. 

Common uses for hex keys

Hex keys are versatile tools used in various applications, including assembling furniture, repairing bicycles, and working on automobiles. They provide increased torque and control compared to traditional screwdrivers, making them ideal for tightening and loosening fasteners in tight spaces.

Metric and SAE Unit Systems

History and origins

The metric system, also known as the International System of Units (SI), was developed in the late 18th century and is now the most commonly used system of measurement worldwide. It uses base units such as meters for length, kilograms for mass, and seconds for time.

SAE units, on the other hand, are part of the United States customary system, which has its roots in the British Imperial System. SAE measurements typically use inches, feet, and pounds as their base units.

Usage in DIY projects

Both metric and SAE unit systems are used in various DIY and home improvement projects. Typically, products made in the United States or designed for the American market utilize SAE fasteners, while those made in Europe, Asia, and other parts of the world use metric fasteners.

Comparing Metric and SAE Hex Keys

Size differences

Metric hex keys are measured in millimeters (mm), while SAE hex keys are measured in fractional inches. The most significant difference between the two systems lies in the increments between sizes. Metric hex keys tend to have smaller increments, allowing for a more precise fit with fasteners.

Pros and cons of each system

Metric hex keys offer several advantages, including:

  • Greater precision due to smaller size increments 
  • Wider global usage, making it easier to find replacement parts and tools 
  • Simplified calculations and conversions due to the base-10 nature of the metric system 

SAE hex keys also have their benefits, such as:

  • Compatibility with American-made products and fasteners 
  • Familiarity for those accustomed to using inches and fractions in measurements 

However, there are also drawbacks to each system. For example, using an SAE hex key on a metric fastener can result in damage to both the tool and the fastener. Similarly, using a metric hex key on an SAE fastener can lead to an improper fit, causing the tool to slip and potentially cause injury.

Choosing the Right Hex Key Set for Your Needs

Assessing your project requirements

When deciding between metric and SAE hex keys, it's essential first to determine the specific requirements of your project. Consider the origin of the products and materials you'll be working with, as this will often dictate whether you'll need metric or SAE tools.

Factors to consider when purchasing hex keys

When selecting a hex key set, consider the following factors:

  • Material quality: Look for hex keys made from high-quality steel, such as chrome-vanadium or alloy steel, to ensure durability and longevity. 
  • Size range: Ensure the set includes all necessary sizes for your projects. 
  • Long and short arm options: Having both long and short arm hex keys provides versatility in reaching tight spaces and applying greater torque when needed. 
  • Storage case: A well-organized storage case helps keep your hex keys in order and prevents loss. 

Related Article: DIY Hex Head Screws

Examples of Metric and SAE Hex Key Applications

Furniture assembly

Many flat-pack furniture pieces use hex bolts and screws for assembly. Products designed for the American market may use SAE fasteners, while those made elsewhere will likely use metric fasteners. It's crucial to have the correct hex key size to avoid damaging the fasteners and ensure a secure fit.

Bicycle maintenance

Bicycle components often use a mix of metric and SAE fasteners, depending on the manufacturer. For example, American-made bicycles may use SAE fasteners for some components, while European or Asian manufacturers may use metric fasteners. Owning both sets of hex keys ensures you're prepared for any maintenance task.

Automotive repairs

Automotive repairs typically require a mix of metric and SAE tools, as different manufacturers use varying fastener types. American-made vehicles often use SAE fasteners, while European and Asian vehicles generally use metric fasteners. Having both sets of hex keys on hand will allow you to tackle any repair job with ease.

Tips for Using and Maintaining Hex Keys

Proper use and storage

To ensure the longevity of your hex keys, it's essential to use them correctly and store them properly. When using a hex key, make sure it's fully inserted into the fastener to prevent stripping or damage. Additionally, avoid using excessive force or leveraging the tool in a way that could cause it to bend or break.

When storing your hex keys, keep them organized in a case or holder that prevents them from becoming lost or damaged. Avoid exposing them to moisture or corrosive substances, as this can lead to rust and degradation.

Preventing wear and tear

Regularly inspect your hex keys for signs of wear, such as rounded edges or bent arms. If you notice any damage, replace the tool immediately to avoid further issues. Additionally, consider investing in a set of hex keys with a ball-end design, which allows for better contact with fasteners and reduces the risk of wear and tear.


When it comes to choosing between SAE and metric hex keys, the decision ultimately depends on your specific needs and the types of projects you'll be undertaking. By considering the origin of the products and materials you're working with, assessing your project requirements, and understanding the pros and cons of each system, you can make an informed decision on which hex key set is best suited for your toolbox.

Equipped with the right tools, you'll be prepared to tackle any DIY project with confidence and ease, ensuring successful outcomes and a sense of accomplishment in your endeavors.

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