One kilogram is equal to 2.2 pounds. According to the National Institute for Standards and Technology, the value of the kilogram has remained close to constant over the centuries, though the exact method used to define it has changed dramatically.
According to the same source, the first value given for the kilogram was set at the end of the 18th century as the mass of 1 cubic decimeter of water. A century later, the kilogram was redefined as equivalent to the mass of a small ball of platinum-iridium alloy kept by the International Bureau of Weights and Measures. The kilogram remains the only metric unit that is still defined by reference to a prototype of this kind, as the meter is defined in reference to wavelengths of light and the liter in reference to cubic centimeters.