What is a Kilogram?

What is a kilogram?

Mass describes how much matter an object contains and how strongly it resists a change in motion. The kilogram is the SI base unit used to report that quantity.

Kilograms are practical for many everyday values: people, food packages, luggage, building materials and vehicle payloads. Grams and milligrams are usually more convenient for smaller values, while metric tons are used for large masses.

The symbol kg is written in lowercase. Write a space between a number and the symbol, such as 5 kg, and do not add a plural ending to the symbol.

The modern definition of the kilogram

The kilogram is defined by fixing the numerical value of the Planck constant, h, at exactly 6.62607015 × 10⁻³⁴ joule seconds. Together with the definitions of the meter and second, this defines the unit of mass.

This definition does not depend on a single physical object. National metrology laboratories can realize the kilogram through experiments traceable to fundamental constants, rather than relying on one stored metal prototype.

For normal weighing, this scientific definition is invisible: a calibrated scale and suitable reference masses provide the practical measurement chain. It matters because it keeps the worldwide mass standard stable and reproducible.

Kilograms, grams and metric prefixes

The kilogram is unusual among SI base units because its name already contains the prefix kilo. To form smaller or larger mass units, prefixes are attached to gram, not kilogram: 1 mg is one milligram, not one microkilogram.

One kilogram equals 1,000 grams, one gram equals 1,000 milligrams, and one metric ton equals 1,000 kilograms. These exact decimal relationships make conversions within the metric system straightforward.

Mass is not weight or force

In daily speech, a person's "weight" is commonly reported in kilograms. In physics, the kilogram is mass, while weight is the gravitational force acting on that mass and is measured in newtons.

Near Earth's surface, a 1 kg mass experiences a weight force of roughly 9.81 newtons. The mass stays the same on the Moon or in orbit, while the weight force changes with local gravity.

Do not substitute kilograms for newtons in an engineering force calculation. If a drawing, test report or specification says kgf, it refers to kilogram-force, which is a distinct force unit.

Kilograms and pounds

Pounds are common on US consumer labels, body-mass discussions and shipping documents. One kilogram is approximately 2.20462 pounds, while one pound is exactly 0.45359237 kilograms.

For a quick comparison, convert the value using the stated unit and retain an appropriate number of decimal places. A unit conversion changes the label of the same mass; it does not account for packaging, tare mass or scale uncertainty.