While the majority of bolt-type fasteners used in the United States conform to one or the other measurement scale, the difference between SAE and metric fasteners (and their corresponding sockets) is as profound as the difference between yards and meters: They are almost the same, but not quite. Each represents a different standard that may or may not have a (nearly) matching size in the other.
What is SAE?
The origin of the SAE standard comes from the English, or Imperial, system, which is based on inches and the fractions thereof. SAE, which stands for the Society of Automotive Engineers, was used as the standard primarily on U.S.-made cars and trucks through the 1970s. SAE sockets are sized in inches and fractions of inches.
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Metric is the preferred fastener measurement throughout the world. More exact than fractions of an inch, metric measurements--based on millimeters--are a precise representation of the diameter of the head of the fastener. Imported cars have used metric bolts for more than 30 years.
Both SAE and metric fasteners have six-sided (hex) heads that are turned using open-end, boxed or socket wrenches. Socket wrenches maintain constant contact with all sides of the bolt head and feature a ratchet system for easier and faster operation in either direction.
One For The Other . . . Sometimes
While there is no precise correlation between SAE fractional and metric measurements, there are many sockets that are close enough in size to be used interchangeably, in some circumstances. For example, 5/16 and 8 mm, 7/16 and 11 mm, 5/8 and 16 mm, 3/4 and 19 mm, 7/8 and 22 mm, and 15/16 and 24 mm.
While car and truck maintenance seems to lead the way on the transition from SAE to metric measurement standards for fasteners, and therefore the sockets used to tighten and loosen them, these same standards are used on virtually all bolt fasteners commonly distributed throughout the world. Many reasonably priced socket wrench sets have both SAE and metric sockets that will cover most repair bases.