Kilogram (kg)

The kilogram is the unit of mass for most of the world, from a bag of rice to a person's body weight. It is one of the seven base units of the International System of Units (SI), and the only one whose name already carries a prefix (NIST SP 330).

At first, the kilogram was meant to be the mass of a cubic decimeter of water at its densest. From 1889 until 2019, it was the mass of a single platinum-iridium cylinder kept at the International Bureau of Weights and Measures (BIPM) near Paris (NIST Handbook 44, Appendix B).

Today it is defined by a fixed value of the Planck constant, a constant of nature, so the unit no longer depends on one metal object (BIPM).

The symbol is kg, with a lowercase k, because prefixes smaller than mega take small letters; a capital K is the kelvin. Unit symbols never take a plural s or a period, except at the end of a sentence, so it is kg for one kilogram or fifty (NIST).

Put a space between the number and the symbol, and write a zero before a decimal point, as in 0.5 kg. Do not mix a symbol with a spelled-out unit name in one expression, such as "kilograms per L."

A liter of water weighs 1 kilogram, which makes a full liter bottle a handy reference. A US quart of water is a little lighter, at 0.95 kilograms.

In the kitchen, a kilogram is about 8.8 sticks of butter. It is also enough all-purpose flour to fill about 8 US cups, based on USDA data. The same weight of granulated sugar fills only about 5 cups.