Introduced in 1791, the metric system is an international system of measurement that replaced the standard (or imperial) system of measurement in most of the world. Since both the metric and standard systems are used in the United States, however, it's likely that you'll need to convert standard measurements to metric, or vice versa, when using tools such as sockets (attachments that fit onto socket wrenches to grip bolt heads). While it's always best to use metric sockets on metric bolts and standard sockets on standard bolts, it's possible to convert a standard socket to a metric size that is close enough to work.
Look at the standard socket you wish to convert; it will show its size in fraction form. Make a note of the size with a pencil and paper for future reference.
Type the numerator (top number) of the fraction into your calculator and divide it by the denominator (bottom number) to change the standard socket size to decimal form. This will make calculations easier. For example, with a 1/4-inch standard size socket, you would divide the 1 by the 4 and get a result of 0.25 inches.
Type the resulting decimal figure from step two into your calculator and multiply it by 25.4. This is done to arrive at a figure in millimeters, because one inch equals 25.4 millimeters. The standard size of 0.25 inches multiplied by 25.4 equals a metric size of 6.35 millimeters, for example.
Round the resulting metric figure from step three up to the nearest metric socket size: 6.35 millimeters rounds up to a metric size 6.5 mm socket.