What is the most convenient way of measuring distance, or time, or any other physical variable? Well, there must be a uniform way of measuring, specially when trading among nations, no matter the language or culture. That's the purpose of the International System of Units
This EBook, prepared by the National Institute of Standards and Technology (NIST) is the English version of an international committee: the International Bureau of Weights and Measures, Le Système International d’ Unités (SI).
The first three of the SI base units are: length, measured meters; mass, measured in kilograms; and time, measured in seconds. Thus, distance in feet, miles, etc. is not only avoided, but not even mentioned. Kilograms substitute ounces, pounds, etc.
In this metric system every unit has a precise definition. For example, take the definition of what is the meter unit of distance: The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. And what is a second? The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. Wow, that is precision! However, when it comes to the definition of mass, measured in kilograms, the definition is not so esoteric nor philosophical: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. That international prototype mentioned here is not a physical constant. The kilogram is a physical amount of matter kept under very strict and controlled atmospheric conditions.
For a simple infographic about the standard kilogram, download How heavy is a kilogram?
Download your free copy of The International System of Units (SI) 2008 Edition. It also includes handy tables of conversion and prefixes, like: deka, hecto, kilo, mega, giga, tera, peta, exa, zetta, yotta, deci, centi, milli, micro, nano, pico, femto, atto, zepto, and yoct.