Conversion between metric and English measures is a useful skill for any citizen of the world to acquire. This guide will help you know a little more about the pound, the kilogram and how to convert one into the other.

There are actually a number of different kinds of pounds. If the old joke "which weighs more, a pound of feathers or a pound of lead" used wool instead of lead, the joke might just be on the joker; the "avoirdupois" pound, which is most commonly used today, is actually lighter than the "wool" pound, which was historically used for the weighing of wool. The "troy" pound weighs less than the wool but more than the avoirdupois pound, and was declared illegal in 1878. It is, however, still often used in weighing gold. The "tower" pound was used for weighing coins and was abandoned in 1527 and the "london" pound died out in the middle of the 14th century. When people refer to a pound, they are most commonly referring to the avoirdupois pound.

The kilogram (kg) is the only unit of measure in the metric system that is still measured in terms of a physical artifact. By contrast, the formal definition of a meter is the distance traveled by light in a vacuum in 1/299,792,458th of a second. There is actually a "prototype kilogram" kept at the Bureau International de Poids et Mesures (BIPM) made of platinum-iridium that has been kept under very specific conditions since 1889 when it was first sanctioned as the official prototype. The kilogram is actually a unit of mass, not weight. The mass of an object is constant and doesn't change; the weight of an object is its mass times gravity, so it can change depending on the amount of gravity present. The continued use of a physical object is problematic though, in 1980 the prototype was compared with its copies from around the world and many of the copies have gained mass. So right now scientists are unsure of exactly how to define the mass of a kilogram. There is a proposed solution to use Avogadro's constant, the ratio of molar mass to the mass of an atom, to define the kilogram since Avogadro number of carbon-12 atoms weigh exactly 12 g. Therefore a kilogram could be defined as the mass of 1000/12 times Avogadro number carbon twelve atoms. However, we are still unable calculate Avogadro's number accurately enough, so the prototype kilogram is still used today.

Compared to the large amount of history and science behind the pound and the kilogram, converting between the two is relatively simple. Because kilograms are a measure of mass and pounds can be a measure of mass or weight, make sure you are using pounds in terms of mass. If you're not, simply divide by 9.8 and you'll have the mass in pounds. From there, the conversion factor is 1 pounds is equal to .4535 kilograms.