Key Takeaways

  • SAE sockets use the imperial system and are used on American-made vehicles.
  • MM sockets are used on European or other vehicles and use the metric system.
  • It’s best to avoid using SAE and MM components interchangeably to avoid stripping or ruining the bolts.

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Whether you're a seasoned mechanic or just getting started, you'll find one decision pops up repeatedly: SAE or MM (metric) sockets?

Both SAE and MM are reliable, so no one is better than the other. SAE is used in American-made vehicles, and the sizing of the bolts, wrenches, and components use the imperial system. Metric sockets use the metric system on European or other regional vehicles.

After using both of these components through the years with many different vehicles, I’ve become an expert on the topic. I will discuss the advantages and disadvantages of each system to help you determine which one might be better suited to your needs. Keep reading to learn more.

Table of Contents

SAE Vs. MM Sockets - Which Is Better? (Complete Guide)

The right socket set can distinguish between a smooth project and a tedious, frustrating endeavor. The type of socket used typically depends on where you are located and where your vehicle was manufactured.

A critical part of making the correct choice lies in understanding the difference between SAE and MM sockets, the two main measurement systems that sockets adhere to. There is no way to say which is better.

Each socket type is used for a specific vehicle design. SAE sockets are used for American-made vehicles. MM or metric sockets are used in Europe and other regions. Let’s review SAE and metric sockets in more detail.

What Is SAE?

SAE is the Society of Automotive Engineers. This organization was established in 1905 and is responsible for setting the standards for the design, manufacture, and operation of various types of vehicles, including cars, airplanes, and boats.

However, its reach extends beyond the automotive industry to include commercial vehicles, off-highway equipment, and even self-propelled vehicles used in space exploration.

When it comes to tools, SAE sockets are based on the imperial or inch-based measurement system. They're commonly used in the United States and are measured using fractions like 1/4-inch, 1/2-inch, 3/4-inch, etc.

These sockets are widely utilized when working on American-made vehicles and equipment. Vehicles with SAE sockets also need SAE wrenches.

What Is MM?

MM, on the other hand, refers to the metric measurement system based on millimeters. Metric sockets have sizes such as 8mm, 10mm, 12mm, and so on.

They are the standard socket measurement system in countries other than the US and are typically used when working on vehicles and equipment manufactured outside the United States.

The metric system was developed in France in the late 18th century and has been adopted worldwide as a universal system of measurement, with the exception of the United States, which predominantly uses the imperial system.

In the context of tools such as sockets, MM sizes refer to the sizes of these tools measured in millimeters according to the metric system. For example, a 10 MM socket would fit a bolt with a head 10 millimeters across.

SAE or MM: Which Is The Better Choice?

When determining which socket size is better, SAE or MM, the answer isn't necessarily black and white. The choice mainly depends on the task you're tackling and the equipment or vehicles you're working on.

If you're working on American-made vehicles or equipment, you may find SAE sockets more suitable. However, if you're handling international vehicles or equipment, MM sockets could be a better choice.

Ultimately, having both SAE and MM sockets in your toolbox would be ideal, as this ensures that you'll always have the appropriate socket size for any job, regardless of the make or origin of the equipment you're working on.

What Are the Differences Between SAE & Metric Sockets?

While SAE and metric sockets may seem similar at first glance, crucial differences between them make each type more suitable for certain applications. Let's explore these differences.

Measurement System

The most fundamental difference lies in the measurement systems they adhere to. SAE sockets are measured in inches and their fractions, which adhere to the Imperial system, also known as the English or U.S. customary units.

Metric sockets, on the other hand, are measured in millimeters, in line with the Metric system. The measurements can sometimes match up, but the different measurement system makes it tricky to find the right metric socket sizes if you have SAE tools and vice versa.


Metric sockets can offer more precise sizing. This is because the Metric system, which measures in millimeters, has a smaller base unit than the Imperial system, which measures in inches.

This increased precision can result in a better fit and less chance of stripping the bolt or screw head. The use of metric bolts has also been done for much longer, and you can use impact sockets to improve precision too.

Size Range

The sizes in each set are slightly different, too. SAE sockets typically come in sizes such as 1/4", 5/16", 3/8", 7/16", 1/2", etc. Metric sockets, on the other hand, would come in sizes like 6mm, 7mm, 8mm, 9mm, 10mm, etc.


Some SAE and Metric sockets are close enough in size that they can be used interchangeably in some situations. However, this isn't ideal as it can lead to damaged fasteners or sockets over time.

If you’re not careful, you could strip equipment if you tried to use an SAE socket on a metric bolt. But you will notice that metric sockets still use imperial-sized drivers.


Another difference lies in their regional usage. In the United States, SAE sockets and fasteners are predominantly used, while other countries primarily use metric sockets.

This means that, depending on the origins of your tools or the machinery you're working on, you may require a specific type of socket system - even within the same country.

For instance, working on an imported car in the US might require metric sockets, while a domestic car would more likely need SAE sockets.

Fastener Compatibility

The fasteners (nuts, bolts, bolt head, screws, etc.) will be designed for either SAE or Metric sockets, so having the right socket type can be crucial to avoid damaging the fastener or the tool.

SAE fasteners are measured in inches and fractions of inches. Metric fasteners are measured in millimeters, which applies to all metric tools. Ideally, you should match the socket type with the fastener type for optimal results.

This applies to both metric wrenches and socket wrenches. So if you’re wondering whether you should buy metric or SAE wrenches, consider the compatibility.

SAE vs. Metric Conversion Chart

We've put together a handy SAE and metric wrench conversion chart to help you navigate the differences between these two types of wrenches and sockets.

It's essential to understand the conversion between these sizes, as it can significantly affect the compatibility and efficiency of your toolset.

In sockets, SAE (Society of Automotive Engineers) sizes are based on inches, while metric sizes are measured in millimeters. Knowing this, let's delve into some of the most common conversions for various socket sizes:

SAE (inches) Metric (millimeters)
1/16 1.5875
1/8 3.175
3/16 4.7625
1/4 6.35
5/16 7.9375
3/8 9.525
7/16 11.1125
1/2 12.7
9/16 14.2875
5/8 15.875
11/16 17.4625
3/4 19.05
13/16 20.6375
7/8 22.225
15/16 23.8125
1 25.4

Can You Use Metric Fasteners On An SAE Socket?

We might encounter several issues when using metric wrenches on SAE bolts. First and foremost, the fit might not be perfect, which could lead to the wrench slipping off the bolt head.

This slippage could potentially cause damage to both the wrench and the bolt head, making it more difficult to remove or tighten the bolt. In some cases, using a metric wrench on an SAE bolt might work, but this isn't guaranteed.

It's important to remember that metric and SAE measurements have different increments, with metric being accurate to within 0.1mm, while SAE measurements can be off by as much as 0.5 inches.

Another potential problem is that using the wrong wrench could damage or strip the bolt head. A stripped bolt head could be difficult to remove, which leads to additional time and effort spent on the project.

How Do You Tell If a Component Is Metric Or SAE?

Identifying whether a nut is metric or SAE, also known as standard or imperial, can be confusing. But don't worry, we're here to help you with some handy tips.

To determine if a nut is metric or SAE, we can use a few simple methods:

  1. Look for markings: Some nuts have markings that can help you identify if they are metric or SAE. For instance, metric nuts may have numbers on them, such as 8.8, 10.9, or 12.9, indicating their grade, while SAE nuts might have marks like three or six radial lines.
  2. Measure the size: Use a ruler or a caliper to measure the size of the nut's flat sides. If the measurement is in inches and fractions, it's most likely an SAE nut. However, if the size is in millimeters, it's a metric nut. Keep in mind that there might be some overlap in sizes. For example, a 1/2-inch SAE wrench reasonably fits a 13mm metric nut.
  3. Check the thread pitch: Examining the thread pitch is another way to tell if a nut is metric or SAE. As mentioned earlier, SAE fasteners have threads per inch (TPI), meaning there are more threads in an inch compared to metric ones. You can use a thread pitch gauge to measure this.

It's essential to ensure you're using the correct metric or SAE tools and fasteners to avoid damage and maintain the integrity of your connection. Remember, forcing a metric bolt into an SAE nut or vice versa can strip the threads and cause issues.

SAE Vs MM - Which Is Better?

About The Author

Charles Redding

Charles Redding

I've spent many years selling cars, working with auto detailers, mechanics, dealership service teams, quoting and researching car insurance, modding my own cars, and much more.

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