Six pounds equals 2.72 kilograms. One kilogram is 2.20 pounds.
Kilograms belong to the metric system, which is based around groups of 10. Each type of measurement (such as mass or distance) has a base measurement. Generally, the base unit doesn't have a prefix; for instance, the base unit for distance is the meter, and the prefixes denote how much larger or smaller the unit is than the base unit. However, for mass, the base measurement is actually the kilogram. A kilogram measures mass, and it's based on the weight of 1 cubic cm of water. Today, even the U.S. pound (the avoirdupois pound) is defined in relation to the kilogram because the kilogram is measured against an actual cylinder, held by the International Bureau of Weights and Measures. Generally, pounds are used in the U.S.; however, as the metric system is more precise and easier for calculations, scientists generally use it even in the U.S.