If you live in the United States or in a country where metric system is not common, you might not understand how other people live in the metric world. If fact metric system is very straightforward and easy, you just need to get used to it.
To make a couple of conversions from or to metric you don't have to understand how it works. Simply pick what you want to convert in the menu on the left and get your result. However, if you are curious and want to know the principles of metric system, read on.
Metric System Principles
The metric system was devised in France in the 18th century. The goal was to replace the chaotic collection of units being in use with a measurement system based on one common standard and simple decimal factors.
The standard for length was defined equal to one ten-millionth of the distance between the North Pole and the Equator. The resulting length was named meter. The definition of meter changed several times since to make it more precise. The latest and the most precise definition of meter is "the distance light travels in a vacuum in 1/299,792,458 second". The standards for other measurements in metric system were set in a similar way.
Initially all base units of metric system had their prototypes – physical objects that represented the canonical length of a meter, a canonical mass of a kilogram, etc. The original prototypes were kept in a secure place in France, precise copies were distributed among participating countries. These physical prototypes had a number of disadvantages though. First, it is hardly possible to make copies for distributing that will be exactly the same. The still could be a subtle difference. This difference might not be important for everyday usage, but for precise scientific calculations it could play a role. Second, no physical object in our universe is eternal. Even the platinum-iridium cylinder that used to be a prototype of a kilogram loses some mass due to minimal evaporation of molecules from its surface.
That's why General Conference of Weights and Measures (CGPM) for years discussed moving from material prototypes to theoretical definitions of units. In 1960 the meter was redefined in terms of wavelength of light. Now every since laboratory in the world could "re-create" the exact meter. A physical prototype was no longer required.
The last surviving physical prototype of a base unit was the one for kilogram. In 2014 CGPM discussed redefining the last unit with a prototype, but decided the data was not sufficiently robust yet. Finally, in the end of 2018 the decision has been made officially. On May 20th, 2019 the physical prototype of kilogram was retired. Now all units in the International System of Units are defined via physical constants and no longer require physical prototypes.
The metric system or International System of Units (SI) is based on seven base units for seven base quantities that are mutually independent. The quantities are: length (meter), mass (kilogram), time (second), electric current (ampere), thermodynamic temperature (kelvin), amount of substance (mole) and luminous intensity (candela). All other units are derived from the base ones.
For example, area and volume are derived measurements that are based on length. As such, they do not require their own independent units. For historical reasons metric system still has units like liter (volume), that exactly equals to one cubic decimeter, or hectare, that is equal to 10,000 square meters. As you can see, all metric units of area and volume are officially defined through meter, the basic unit of length.
New units of any measurement can be constructed from the corresponding base unit by adding one of metric prefixes. The table of metric prefixes useful for metric conversions is provided below.
Metric prefixes are easy to understand and very handy for metric conversions. You don't have to know the nature of a unit to convert, for example, from kilo-unit to mega-unit. All metric prefixes are powers of 10. The most commonly used prefixes are highlighted in the table.
By the way, you can make online conversions for metric prefixes on our Fractions and Percent page.
Most people even in the countries where metric system is used only know the most important metric prefixes like 'kilo' and 'milli'. These are highlighted in the table above. Other prefixes like 'zepto' or 'yotta' are very specific and used mostly in science.
It may be worth noting that not all the above prefixes existed since the very beginning. The oldest ones are 'kilo', 'hecto', 'deca' and thee their counterparts 'milli', 'centi', and 'deci'. These six prefixes were adopted back in 1795. The pair of 'mega' and 'micro' was added almost a century later, in 1873. Six pairs more were added in the 20th century. And only recently in November, 2022 two last pairs were added: 'ronna', 'quetta', 'ronto', and 'quecto'. Now we can say that the mass of the Earth is about 6 ronnagrams.