The metric system's real trick is that its units need no conversion factors
Decimal prefixes like kilo and milli get the credit, but the metric system's deeper design is coherence. Its derived units are built purely from products of base units, with no stray multipliers. A newton is just kilograms times metres per second squared. Almost every country has adopted it, with one notable holdout.
A system of measurement is a framework for expressing physical qualities as numbers. A metric system fixes a set of base units and uses decimal prefixes for bigger and smaller amounts: kilo means a thousand times, milli a thousandth, so a kilometre is 1,000 metres and a milligram a thousandth of a gram. The idea grew from the metre, introduced in France in the 1790s, and culminated in the International System of Units, or SI, defined in the mid-20th century under an international standards body.
The SI today rests on seven base units: metre, kilogram, second, ampere, kelvin, mole and candela. Everything else is derived. The hertz is cycles per second, the tesla combines kilograms, seconds and amperes, and the square metre measures area. The Celsius degree is a shifted version of the kelvin, with 0 degrees Celsius equal to 273.15 kelvin but identical step sizes. All base units are now defined through physical constants rather than prototype objects, as they once were.
Its design reflects principles hammered out over two centuries. Choose independent dimensions of nature as base quantities, assign each a base unit, prefer natural phenomena to physical artefacts, and derive everything else by logic rather than experiment. Time should be measured in seconds, length in metres or decimal multiples, and mass in grams or decimal multiples.
The SI was not the only branch. It descends directly from the metre-kilogram-second system, while physicists also built the centimetre-gram-second family, whose Gaussian variant remains popular in electromagnetism, plus a metre-tonne-second system and gravitational systems based on force. Some non-SI units, notably the litre, stay in everyday use. Switching to the metric system is called metrication, and nearly every nation has done so officially. The United States uses metric units in some settings but has resisted full adoption and keeps other systems alongside it.
Source: Metric system
