You can purchase a metric socket wrench set almost anywhere. Their common sizes and flexibility make them nearly ubiquitous. Metric and SAE (Imperial or English) sockets are often combined into a single set to ensure that you have the correct size socket for any bolt, nut or screw you may find. These sets may include a spinner handle and other accessories.
The ratchet handle connects to many different sizes of sockets. It provides leverage that makes tightening or loosening nuts, screws or bolts easier. A square post in the ratchet head fits tightly into a square hole in the socket. The measure of this post yields the drive size.
Drive sizes come in dimensions of 1/4 inch, 3/8 inch, 1/2 inch, and 3/4 inch. Combination sets usually include 1/4-inch and 3/8-inch drive sizes. The 1/2-inch and 3/4-inch drive sizes, commonly used for higher torque (power) applications, usually do not come in sockets smaller than their respective drive sizes.
Metric socket sets contain several different sizes of sockets. Certain size sockets appear in most metric (and combination) socket wrench sets. These sizes are 3mm, 4mm, 5mm, 5.5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm, 12mm and 13mm.
Metric sockets 14mm, 15mm, 16mm, 17mm, 18mm, 19mm, 20mm, 21mm, 22mm, 23mm, 24mm, 27mm, 30mm, 32mm, 34mm, and 36mm are available singly, in standard sets, or by special order.
Number of Points
Common sockets fit hex head bolts and nuts. A six-point socket contains the minimum number of points for driving hex head bolts and nuts. A 12-point socket gives greater flexibility when fitting the socket over a bolt or nut. A 12-point socket will round or otherwise deform using less torque than a six-point socket made of the same metal and thickness.
Higher quality sockets constructed of harder, more expensive metal will last longer than less expensive sockets. Higher quality sockets also endure higher torque better than less expensive sockets. Therefore, they loosen nuts and bolts that less expensive sockets cannot.
If two sockets are made with the same kind of metal, the thicker socket will be stronger and deliver greater torque. Heavier thickness equals more expense. A thicker socket may be too big to fit in a confined space, although this is rarely an issue.