If you are a native or even a longtime resident of the United States, you've probably internalized two basic things about the ​metric system​: The rest of the world uses it as the primary system of measurement for essentially everything that is measurable, while the U.S. for the most part does not.

If you're from outside the U.S., it would be reasonable for you to wonder what the holdup there is; after all, the metric system is "obviously" superior to all other measuring systems, some of which feature units that are quaint beyond description.

The metric system is, for the most part, a model of exquisite mathematical symmetry and simplicity. It's not hard to explain why scientists use the metric system for scientific measurements; by structuring the units pertaining to a given physical ​quantity​ (e.g., length, mass or temperature) around successive powers of 10, the system's different levels of magnitude make a modicum of intuitive sense. (What's easier to do in your head, convert 10 kilometers to meters or convert 10 miles to feet?)

The metric system is the international system of weights and measures. It is used universally in the scientific community, but to say that it has failed to catch on in the United States would considerably understate that nation's reluctance to adapt in this area. As an American, when is the last time you can recall buying a known number of liters of gasoline? Do you know your own height in meters or your mass in kilograms?

The metric system is a ​decimal system​ – that's the technical term for anything pertaining to the Arabic system of numerals, 0 through 9, used the world over. In this system, when you move a number's decimal point (the "period" in a number) one place to the left or to the right, you divide or multiply that number by 10 respectively.

A decimal point can be placed at the end of a number that lacks one, and as many zeroes placed to the right of it as you wish, without changing its value. This can be helpful when preparing to make conversions between metric units: For example, 1 km = 1.000 km = 1,000 m, because 1 km = 10³ m.

The metric system was first officially adopted in France in 1795, with special emphasis placed on the ​metre​, or meter (m), and the kilogram (kg). The system's geographic roots explain why "International System" is abbreviated "SI" – in French, this is ​Système​ ​Internationale​.) After the French Revolution in 1789, scientists desired a less cumbersome way of converting between units of the same quantity.

Looking at modern non-metric length units alone, consider how odd it is that a foot has 12 inches, a yard has 3 feet, a furlong has 220 yards and a mile has 8 furlongs. If someone asked you to convert 9.25 yards to smaller units, you'd be expected to include both feet and inches along with a fractional remainder if needed. In this case,

But how many inches is 0.75 feet? Multiplying this number by (12 in/1 ft) gives 9 inches, so the answer is 27 ft 9 in. Not "rocket science," but also not convenient!

It was smartly decided that a physical constant that would presumably never change be selected as a base unit. ​The distance equal to 1/10,000,000 of the distance from either pole to the equator​ was selected, a distance now known as the ​meter​.

• The meter is the starting point for a variety of other metric units. For example, the standard unit of mass, the kilogram, was chosen to represent the amount of matter in exactly 1 liter (L) of pure water, which is 1/1000 of a cubic meter (m³). 

The metric system has seven basic units of measurement. "Basic" means that the power of 10 implied is the standard-bearer for the whole range for that quantity. This is usually either for historical reasons or because the basic unit corresponds to something in most people's common experience. These are, with further details:

​Length – meter (m):​ This is a measurement of pure distance, as in "How far is it from New York to London?" or displacement of an object, as in "How far did you go in flying from New York to London?" The modern scientific standard is based on the speed of light in a vacuum, not a portion of the Earth's surface.

​Mass – kilogram (kg):​ Formerly defined as the mass of 1 cubic decimeter of water, making 1 liter (L) of water equal to 1 kilogram (kg), the modern definition was determined using "atomic" criteria.

​Time – second (s):​ This essential quantity allows for the definition and computation of displacement (m/s) and acceleration (m/s²). Its inverse, cycles per second, is essential in the study of electromagnetic waves, and the unit for this is hertz (Hz).

​Amount of substance – mole (mole):​ One mole (mol) of any substance contains exactly 6.02214076 × 10²³ basic units. This number is essentially the basis of modern chemistry and owes its origin to the properties of the element carbon, 1 mol of which has a mass of precisely 12 grams (g).

​Electric current – ampere (A or amp):​ This represents the amount of ​electric charge​ moving past a point in space per unit time. 1 A is equal to a flow of one fundamental unit of charge (i.e., that on a proton or an electron) per second.

​Temperature – kelvin (K):​ The basic unit of temperature measurement is also the most obscure. It was chosen because its zero point represents the lowest possible theoretical temperature. It is actually the Celsius (C) scale shifted upward by 273 degrees, or 0 degrees Celsius = 273 K.

• Unlike the Celsius and Fahrenheit (F) scales, which often appear with a degree (°) symbol, K is not coupled to a degree symbol.

​Luminous intensity – candela (cd):​ This more obscure unit describes the output of objects that emit electromagnetic radiation, such as stars and light bulbs.

Scientists benefit from a common system of measurement so that they can communicate theories, ideas and most importantly data in a way everyone understands, if not intuitively than readily enough. (Some readers may recall the days when different brands of Android phones each had a unique type of USB charging cable, rather than the universal type available now. It's a rough analogy, but most would agree that this industry change has made the world an easier place for all Android users.)

It is virtually impossible to comprehend any modern, data-rich research in the natural or physical sciences without referring to the metric system and being able to contextualize the numbers and units it includes.

U.S. Congress passed the ​Metric Conversion Act of 1975​ in an initial effort to increase use of the metric system in the United States, but did nothing to ensure that it would be adopted; it was more of a "trial balloon." This balloon did not float very high, and today, the primary proponents of the use of the metric system in the U.S. are certain federal agencies and ambitious educators.

A list of common prefixes used in the metric system is available in the Resources. (Interesting trivia: Despite its small value, the pF, or picofarad – one-trillionth of a Farad – is a typical value of capacitance in electrical circuits.)