The function of a socket wrench is to tighten or loosen fasteners such as nuts and bolts. Socket wrenches come with a long handle to which different size sockets can be attached for tightening different size fasteners. If you choose a socket that is too large, you could damage the tool and your hands. Therefore, it's important to choose the precise socket size that fits the particular fastener.

There are four common socket sizes: 1/4 inch (0.6 centimeters), 3/8 inch (0.9 centimeters), 1/2 inch (1.3 centimeters) and 3/4 inch (1.9 centimeters). In-between sizes are available too, starting with 1/4 inch (0.6 centimeters) and increasing every sixteenth of an inch (0.16 centimeters). In addition, sockets are available in various shapes to fit the different bolt heads: six points (hexagon), eight points (double square), and 12 points (double hexagon). The six-point sockets fit ordinary hexagon bolts; the eight-point sockets fit square bolts; and the 12-point sockets fit both hexagon and square bolts. The 12-point sockets offer double positioning, allowing you to attach the wrench quicker; however, there is more of a chance that it will slip off. On the other hand, although the six-point sockets slow you down, they are tighter and more secure.

Since the majority of the world is moving away from the standard socket sizes and toward metric socket sizes, it's important to be able to convert back and forth between standard and metric measurements. Metric socket sizes range from 3 millimeters to 13 millimeters. If you're using a metric nut, it's best to use a metric socket to match. Similarly, when using a standard nut, use a standard socket.