SAE Vs. Metric: Which Wrench or Socket Do I Need?

Ryan Womeldorf
by Ryan Womeldorf

In the United States, the vast majority of wrenches and fasteners are SAE, while other nations use metric tools. However, in this day and age, you may very well need a set of both. This is especially true if you are using your wrenches to work on your imported car. Here are some ways to tell the difference between SAE fasteners and their metric counterparts.

SAE wrenches and metric wrenches look similar at first glance. Both systems have 6-sided heads on the fasteners and you can find open-end, boxed, or socket wrenches in either type. However, SAE and metric wrenches use entirely different measurement systems. Metric tools use millimeters to express their measurements while SAE uses inches and fractions.

So, which wrench and socket should you buy? And, are these tools interchangeable? We’ll let you know some interesting facts about SAE and metric wrenches and which tool you’ll need for your job.

What is SAE?

The SAE measurement system has roots originating from imperial English. It stands for the Society of Automotive Engineers and uses fractions to designate the different socket sizes. These measurements are displayed in fractions of an inch. So, you would have sizes like ½”, 5/8”, 13/16”, etc.

As recently as the 1970s, the SAE measurement system was the standard used in the United States for cars and trucks. Though with the rise of more imported vehicles being sold in the U.S., you will find that the metric system has gained a lot of ground.

What is Metric?

Whereas SAE is quite common in the United States, the metric system is the most common measurement type throughout the rest of the world.

The metric system is based on millimeters, so metric socket sizes are generally sized like 8mm, 10mm, 12mm, and so on. The advantage of the metric system is that you can get accurate measurements down to the very smallest increments. One millimeter is the equivalent of 3/64 of an inch, so you can get pretty exact! The metric system has been the choice for imported cars around the globe.

SAE vs. Metric Chart

When using an SAE measurement, there are charts available to give you the appropriate metric equivalent. For many domestic vehicles, you would want to use SAE as that is the standard in the United States. If you have a metric set, there will be a conversion chart for domestic sizes.

It is important to make your choice based on the requirements set before you. If you can, keep both metric and SAE on hand for any job type. The more precise you can get, the better the fit and the tighter the fit.

SAE vs. Metric: Which One to Buy?

Your needs will depend entirely on the job at hand. Are you working on a foreign product? Then it is very likely you will need a metric wrench and socket. Is it U.S.-made? Then an SAE wrench is a safe bet.

However, there are many products on the market that are just as likely to use one system as the other. You may not even be sure which system your product uses until you start opening it up. This can make it hard to plan ahead. The best course of action is to have both SAE and metric on hand just in case.

Most sets these days come with conversion charts for ease of use. If you want to be really prepared, some sets come in both SAE and metric sizes so that you can be ready no matter what. It all depends on how much you use the socket set.

How to Identify Different Bolts and Nuts?

Depending on what you are working on, you may need to be able to identify the different sizes of nuts and bolts. Manufacturers generally identify screws, nuts, and bolts by things like major diameter, length, type, pitch, class, and so on.

Each of these variables is important as there can be interference with the other parts and won’t fully utilize the bolt’s design. Most bolts and nuts should have information stamped on the face. Usually, you will see bolt type and the tensile strength as well.

Tensile Strength

In some instances, the tensile strength of the bolt may not be indicated on the head. For those with a tensile strength of less than 64,000 PSI, also known as soft bolts, there will be no indication stamped on the head.

  • Grade five bolts. When you see a bolt head that has 3 raised slots stamped on it, that indicates medium carbon steel. This is the minimum commercial quality that comes with a tensile strength of at least 105,000 psi.
  • Grade six bolts. These have been tempered and are of medium commercial quality, containing at least a 133,000-psi capacity.
  • Grade eight bolts. These medium carbon alloy bolts have been tempered and quenched, rated for 150,000-psi, and are considered the best commercial quality.
  • Grade 12 bolts. These are meant for competition purposes and critical use. These have eight raised slots and have a special alloy steel that has been tempered and quenched. These are the top-of-the-line bolts in terms of tensile strength.

Coarse vs. Fine Threads

When using standard inches (generally in the United States), there are two different types of threads: Unified National Fine and Unified National Coarse. Fine vs coarse is defined by the thread pitch which is the distance between the crest of the thread to the crest in the next thread.

Knowing the thread pitch helps you to determine the type of thread. When the pitch is smaller, there are more threads per inch. The larger the pitch, the fewer threads per inch.

  • Coarse threads. You would want to use coarse threads for aluminum and cast iron. The reason is that they won’t strip the mating hole as easily as a screw or bolt with fine threads would. They also screw in and out quicker and generally don’t gall or strip as easily.
  • Fine threads. Fine threads, on the other hand, tend to create more torque. They have a better holding capacity than coarse threads but they are also more likely to strip and gall than coarse threads.

Why are There No Metric Drive Sizes?

Generally speaking, you can find both SAE and metric sockets in three different drive sizes: 1/4”, 3/8”, and ½”. The original ratchet is actually an American invention, though metric sockets still make use of imperial-sized drivers.

The dimensions for the drivers always follow US sizes (the aforementioned sizes plus ¾”). That is a good thing as it means one less set that you have to worry about if you are looking specifically for a drive set. A good rule of thumb is that smaller sockets use smaller drive sizes. So, if you had a 6mm socket, you would be looking at a ¼” drive size.

What are the Most Common Sizes for Metric Sockets?

Metric sizes can range across the spectrum, though there are more common sizes generally available. The most commonly sized socket wrenches, sets, and flare nut wrench sets are between 8mm and 19mm.

Getting more granular, the most common size for car repair is the 10mm socket, though 12mm, 14mm, 17mm, and 19mm are all fairly common as well. Make sure that you have a drive set as well as separate lug nut sockets available if you use socket sets frequently.

Can You Use SAE Sockets on Metric Fasteners and Vice Versa?

The good news is that, yes, you can generally use some sockets for both SAE and metric sizes. Just be extra careful, in most cases, they are not quite the same and you can wind up stripping a bolt by using the wrong corresponding size.

If you can avoid it at all, don’t convert. They can be close enough in most instances, but it is always better to have a metric and SAE set on hand.

Conversion Chart

When in doubt, use the following conversion chart. You will find that, depending on the manufacturer, there are different tolerances and that they may not fit quite the way that they should. The general rule of thumb for SAE and metric sockets that are close or the same size:


Don’t forget that these are just guidelines. You may find that your specific set or wrench does not fit properly onto the bolt. The rule is always “if it doesn’t fit, just quit.”

Impact Sockets vs. Chrome Sockets

Keep in mind that there is a significant difference between chrome and impact sockets. Understanding the difference can save you a lot of time, trouble, and money in the end.

Impact Sockets

Impact sockets, as the name implies, are meant to be used with an impact wrench. These sockets’ walls are 50% thicker than the average hand socket making them very durable. You can use impact sockets on hand tools without a problem.

Chrome Sockets

Chrome sockets, meanwhile, are meant for light impact wrench use or to be tightened by hand. Never use chrome sockets on impact wrenches because they have different torque settings. When you use non-impact chrome sockets that aren’t rated for impact use, you run serious risks. There is the chance that you could break the sockets at best. That means paying for new sockets. At worst, you could be dealing with exploding metal. It should go without saying that exploding metal is a truly dangerous situation.

Should I Buy SAE or Metric Sockets?

In most cases, the question of whether you should buy SAE or metric sockets will depend on your purposes. If you are planning to do some work on a foreign or imported vehicle like Subaru, Toyota, Nissan, Kia, Honda, Hyundai, etc., then you will need metric sockets. For these cars, you would not need SAE sockets.

Though, with this in mind, if you will find yourself working on both domestic and foreign cars, you will need both SAE and metric sockets. Generally speaking, all of the Japanese, German, and Korean brands use metric sockets. Whereas American brands may use all SAE sockets or all metric sockets, some use half of each. For example, some trucks use the metric system for fasteners, while some cars use both SAE and metric fasteners.

While this makes things very complicated it often happens because the engine may be imported but the body is assembled in the United States. So, in other words, if you plan on working on domestic cars, you will probably need both SAE and metric sockets.


Metric wrenches and sockets work the same way as SAE, but they have a different measuring system. Historically the U.S. has used SAE wrenches and sockets, however, many new American vehicles are beginning to use metric fasteners. This is due in part to the increasing infiltration of American cars in foreign markets.

Although SAE has slid in prevalence throughout the years, it is still a good idea to have this type of socket set. This is because many US-made products still use the SAE system. When it comes to cars, though, it is safe to assume that you will need metric fasteners to get the job done properly.

Ryan Womeldorf
Ryan Womeldorf

Ryan Womeldorf has more than a decade of experience writing. He loves to blog about construction, plumbing, and other home topics. Ryan also loves hockey and a lifelong Buffalo sports fan.

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