Kg is the abbreviation for kilogram, the basic unit of mass in the metric system. A kilogram equals 1,000 grams and weighs about 2.2 pounds. A large textbook, a liter bottle of water, or a pineapple each weighs roughly one kilogram.

Scientists around the world use kilograms because the metric system makes calculations simple: everything works in units of ten. Where the customary system uses ounces, pounds, and tons with complicated conversions, the metric system moves smoothly from grams to kilograms to metric tons (1,000 kilograms).

For over a century, the kilogram was defined by a platinum cylinder kept in France called the International Prototype Kilogram. Every country's measurements traced back to this single physical object. In 2019, scientists redefined the kilogram using fundamental constants of nature instead, so it no longer depends on any physical object.

When you see kg on a food package, a shipping label, or in a science experiment, you're looking at one of the most important measurements in the world. Almost every country except the United States uses kilograms as the primary unit of mass for everything from buying groceries to weighing cargo ships.