About Units of Measurement | The SI System, Metric History and Binary Prefixes

The International System of Units

The SI is built on seven base units: the metre, kilogram, second, ampere, kelvin, mole and candela. Every other unit, such as the newton, joule or pascal, is made from these. Since 2019 all seven are defined by fixed constants of nature, such as the speed of light and the Planck constant, so anyone with the right equipment can reproduce them anywhere.

How the metric system began

In the 1790s revolutionary France replaced hundreds of local measures with one decimal system 'for all people, for all time'. The metre was set at one ten-millionth of the distance from the North Pole to the equator, measured along the meridian through Paris. The Metre Convention of 1875 made it international, and the modern SI followed in 1960.

Decimal and binary prefixes

In the SI, kilo always means 1000, mega a million and giga a billion. Computers count in powers of two, so memory makers long used kilobyte for 1024 bytes. To end the confusion, the IEC introduced kibi (Ki, 1024), mebi (Mi) and gibi (Gi) in 1998. A drive sold as 1 TB holds 10^12 bytes, which an operating system counting in binary shows as about 931 GiB.

Imperial and US customary units

Inches, feet, pounds and gallons are still common in the United States and in parts of daily life in the United Kingdom. Since 1959 they are defined exactly in metric terms: an inch is 25.4 mm and a pound is 0.45359237 kg. Volumes differ, though: an imperial gallon is about 4.546 litres and a US gallon about 3.785 litres.

Frequently asked questions

Why was the kilogram redefined in 2019?

Until then the kilogram was a metal cylinder kept near Paris, and its copies slowly drifted apart by tens of micrograms. Now it is fixed through the Planck constant, so it can no longer change.

What is a kibibyte?

A kibibyte (KiB) is exactly 1024 bytes, while a kilobyte (kB) is 1000 bytes. The difference grows with each step: a gibibyte is about 7.4% larger than a gigabyte.

Which countries do not use the metric system?

Almost every country has adopted it officially. The United States still uses customary units in daily life, and Liberia and Myanmar use a mix. Even in the US, science, medicine and the military work largely in metric.