A kilogram is a unit of measurement for weight (or more precisely, mass). One kilogram equals about 2.2 pounds. A small laptop computer weighs about one kilogram, as does a one-liter bottle of water or a large textbook.

The kilogram is the basic unit of weight in the metric system, which most countries use for measuring. Scientists everywhere use kilograms because they make calculations easier: everything works in units of ten. While Americans typically measure their weight in pounds, a doctor might note that a healthy ten-year-old weighs around 32 kilograms (about 70 pounds).

A kilogram is one thousand grams. When abbreviated, kilogram becomes kg, and you'll see this on food packages, shipping labels, and science equipment.

For generations, the official kilogram was a special cylinder of metal kept in a vault in France. Every other kilogram in the world was calibrated against this single object. In 2019, scientists redefined the kilogram using unchanging properties of physics instead, so now the measurement will never vary, even if that metal cylinder somehow changed.