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Know the relationship between both measurements. Convert most units with multiplication. Convert some units with division.
Since U.S. standard units and metric units do not follow the same scale, you'll need to know the mathematical relationship between the unit you have and the unit you need when converting one to the other.  A few standard to metric conversions worth memorizing include:  1 inch = 2.54 centimeters 1 inch = 25.4 millimeters 1 foot = 30.48 centimeters 1 yd = 0.91 meters 1 mile = 1.6 kilometers   A few metric to standard conversions worth memorizing include:  1 millimeter = 0.04 inches 1 centimeter = 0.39 inches 1 centimeter = 0.0325 feet 1 meter = 3.28 feet 1 meter = 1.09 yards 1 kilometer = 0.62 miles When you know how many of the intended unit exists for one of your original units, you can multiply the original value by the conversion factor.   Example 1: Convert 5.4 inches to centimeters.  There are 2.54 centimeters in 1 inch, so: 5.4 * 2.54 = 13.72 centimeters    Example 2: Convert 13.72 centimeters to inches.  There are 0.39 inches in 1 centimeter, so: 13.72 * 0.39 = 5.4 inches If you only know how many of the original unit exists for one of the intended unit, you'll need to divide the original value by the conversion factor.   Example 1: Convert 5.4 inches to centimeters.  There are 0.39 inches in 1 centimeter, so: 5.4 / 0.39 = 13.8 centimeters    Example 2: Convert 13.8 centimeters to inches.  There are 2.54 centimeters in 1 inch, so: 13.8 / 2.54 = 5.4 inches