**Forty pounds is equal to 18.1437 kilograms.** There are 2.2046 pounds in a kilogram. To convert pounds to kilograms, divide the number of pounds by 2.2046.

A kilogram is a unit of measurement for weight or mass. It is 1,000 grams. A kilogram was originally defined as the mass of 1 liter of water at 32 degrees Fahrenheit. In 1889, a platinum-iridium alloy block was manufactured to be used as the official prototype for the weight of a kilogram. The prototype is kept at the International Bureau of Weights and Measures located in France. Until 1799, the unit of 1,000 grams was called a grave.