Kilogram
The kilogram (kg) is the SI base unit of mass (m). A kilogram was originally defined as being the mass of one litre of water. The more modern version of this definition agrees with this original definition within 30 parts per million [1].
A good way to visualize kilograms is to look at it in terms of water. Since litres can be considered in terms of metres cubed, kilograms of water can be compared to litres or metres cubed to better visualize quantity. For example, 1000 kilograms of water would fit in a 1 metre cubed volume or 1 kilogram of water would fit in a 0.001 metre cubed volume. Now consider an Olympic size swimming pool, about 25 metres by 50 metres, that has a depth of 0.8 metres. That volume, which is equal to 1000 metres cubed, would hold 1,000,000 kilograms of water [2].
[3]
Since the kilogram already has a prefix as part of its name, SI prefixes are concatenated with the unit gram instead. For instance, one-millionth of a kilogram is 1 milligram, not 1 microgram. The kilogram is a widely used measurement in science, engineering and commerce world wide [1].