A unit is a standard of measurement, such as the meter, inch, hour, or pound. The two main systems in use are the U.S. customary units (feet, pounds, gallons, etc.) and the SI or metric system (meters, kilograms, liters, etc.). SI stands for Le Système International d'Unites, or the International System of Units. In addition, some special units, such as a square of roofing material, and some obsolete units, such as rods and chains, must occasionally be dealt with.

Most units have an abbreviated form or a symbol, so that we do not have to write the full word. Thus the abbreviation for millimeters is mm, and the symbol for ohms is Ω (capital Greek omega). In this section we will usually give the full word and the abbreviation.

We convert from one unit of measurement to another by means of a conversion factor. Conversion factors for units of measurement, as well as the abbreviations for those units, are given in Appendix B.

## Example 100:Convert 1.530 miles (mi) to feet (ft). Solution: From Appendix B we find the relation between miles and feet. - 5280 ft = 1 mi
Dividing both sides by 1 mile, we get the conversion factor. We know that we can multiply any quantity by 1 without changing the value of that quantity. Thus if we multiply our original quantity (1.530 mi) by the conversion factor (5280 ft/mi), we ... |

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