The metric system is an international standard for measuring, using meters, liters and grams to denote length, volume and mass, respectively. While many world nations have adopted the metric system for official use, the United States has not, as of September 2011. Officially, the United States uses the American system of measurement, though many applications, such as science and mechanics, favor use of the metric system.
Standard tools are made to fit and work on items designed using the American system of measurements, formally called the United States customary system. Generally, the tool size and whatever it is used on is expressed in inches. For example, a 1-inch wrench is designed to be used with a fastener that has a head that is 1 inch in diameter.
Metric tools are designed in accordance with the metric system of measurement. Ideally, these tools are used with fasteners like screws, bolts and nuts that are also designed using the metric system, i.e. a 10-mm wrench and a 10-mm nut. Depending on the tool, it may be more common to find an item measured using the metric system. For example, hex wrenches and L key sizes are often expressed using millimeters, i.e. 1.5-mm tip, 10-mm tip.
When it Matters
The difference between metric tools and standard tools is slight for some applications while in other instances the difference may make a job harder. For example, a tool designed using the standard system may be ineffective against a bolt that is designed using the metric system. In many cases this may result in damage to the bolt and make tightening or removing it more difficult than using a metric-based tool. This is often seen when working with mechanics designed outside of the United States as all of its components are built using the metric system.
Coversion Charts and Micrometers
There may be occasions when you are working with a set of tools that are made with the American standard and materials that are sized using the metric system or the other way around. Matching the correct system of tool with an appropriate nut, bolt or screw is ideal but not always possible. It is useful to keep conversion charts in your toolbox for this scenario as they help you select the most appropriate tool for a particular job. A micrometer is a precision tool used for measuring the kind of small items used to bolt devices together. For example, by measuring the head of a nut, you're able to select the appropriate metric tool or use the conversion chart to find the next best thing in a standard tool.