**A single liter of water weighs precisely one kilogram, or 2.20 pounds.** This is because liters and kilograms have historically been defined in terms of one another.

When the metric system was first developed by a group commissioned by France's Louis XVI, a unit of mass called the grave was defined as being the weight of a block of ice one liter in size. The grave was later renamed the liter by the French Republic after Louis XVI was disposed. In 1889, the kilogram was redefined as being the mass of an example weight of platinum-iridium kept in the International Bureau of Weights and Measures. In 1901, the liter was redefined as being the volume of pure water that has the same weight as that platinum-iridium mass. Since 1964, the liter has been defined as being a volume of water that weighs exactly one kilogram, or 2.20 pounds.