Commonly used units of length in the U.S. include the inch, foot, yard and mile. Twelve inches equal 1 foot. 3 feet equal 1 yard. 5280 feet equal 1 mile. For accuracy in measurement and documentation, smaller objects and distances are measured in inches, feet and yards. Traveling distances are commonly expressed in miles.
The meter is the standard unit of measurement in the metric system or SI. According to the BIPM, the meter is described as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second. In practical terms, the meter is equal to approximately 3.28 feet, 1000 mm equal 1 meter, 100 cm equal 1 m and 1000 m equal 1 km. Small objects are typically measured in centimeters and millimeters. Traveling distances are measured in kilometers.
Unit conversions are necessary for communication between SI and Imperial system users. Knowing some commonly used dimension conversions can be helpful, especially when reading instructions written by SI users: 1 inch equals 2.54 cm, 1 foot equals 30.48 cm, 1 mile equals 1.60934 km. Easy-to-use conversion calculators are available online.
You calculate derived units, such as area, volume, speed and density by multiplying or dividing two or more fundamental units. You find the area of a two-dimensional surface by multiplying its length times its width, the volume of a three-dimensional object by multiplying its length times its width times its height. Speed is calculated by dividing distance traveled by travel time. Density is calculated by dividing the volume of an object by its mass.