A Guide to Socket Sizes: Understanding SAE and Metric


socket sizes

If you’re working on cars in the United States, you’ve probably run into the problem of SAE vs. metric socket sizes. The US is one of only three countries that doesn’t use the metric system, which can make finding the right socket size a serious pain.

Of course, it’s more complicated than saying a foreign car uses metric and an American car uses SAE. Car companies are international brands, and frequently import cars from other countries. You may find a GM car that needs metric sockets.

Ahead, we’ll give you a quick rundown of SAE and metric so you’ll be better equipped to make adjustments to your home car maintenance.

Socket Sizes at a Glance

SAE is the English standard, and the acronym stands for Society of Automotive Engineers. You’ll mostly find this measurement on American made cars, especially older models. SAE measures in inches and fractions of an inch.

Metric is used throughout the world and is far more precise regarding measurement. Millimeters are a far more accurate way to measure fastener heads, and more car companies are moving toward metric as the new standard – even in America.

The only difference between SAE and metric fasteners is the measurement they use. Both types have six-sided heads, but you need to make sure you have the closest fit possible.

How Can You Tell?

Thankfully, the companies that manufacture fasteners make it easy for customers with their markings. You’ll see an ISO (International Standards Organization) marking on metric bolts. This marking usually comes in the form of “ISO M” on the top of the head.

Additionally, you’ll probably see a number on the top of a metric bolt, informing you of the strength. If the number is higher, the bolt is stronger.


There’s no one-to-one conversion when going from SAE to metric (or vise versa). This element is what makes it difficult when figuring out the socket sizes you need. If you use an SAE socket on a metric bolt, you run the risk of stripping or breaking the fastener.

There are some general rules, though, that you can use if you’re in a pinch. 5/16 of an inch is close to 8 mm, 3/8 in is 10 mm, 7/16 in is 11 mm, and so on. We’ve included a chart to make things as easy as possible when trying to do conversions on your own.

Always Have Both

If you’re frequently working with car parts, it’s smart to invest in sockets with both metric and SAE measurements. Metric is becoming more prevalent, but you’ll still need some SAE sockets just in case.

You can probably find an inexpensive socket set with both standards of measurement, so be prepared before you find yourself with a metric bolt and SAE socket.

SAE vs. Metric

Hopefully, this guide gave you everything you need to know about SAE and metric socket sizes. If you get a set of both, you shouldn’t run into any trouble with inaccurate conversions.

If you have any more socket-related questions, take a look at our advice page. We’re always ready to help on anything related to sockets!

Leave a Comment:

Leave a Comment: